Why Does My Car Hesitate To Start? Common Issues And Fixes!

You turn the key like it’s any other day, but rather than turning over with a vigorous vroom, your car’s engine hesitates, and after a few worrisome seconds of trying it finally starts. Obviously, any time a chatters and stalls on start-up it’s a cause for concern.

You’re left to wonder; will it start again if I turn it off? Is this a problem that could leave me stranded just a few miles down the road? Before you panic, let’s try to figure out causes delay in car starting.

The top reasons why your car hesitates to start usually comes down to a battery problem, a problem with the starter motor, or a problem with the fuel system. Though these are just the tip of the iceberg when it comes to reasons why your car is struggling to start. Accurately troubleshooting the cause is a necessary first step in executing the correct fix.

Top Ten Causes of a Car That Hesitates to Start

There are a lot of different things that can cause your car to hesitate to start. Though the top ten most common reasons that cause this problem are:

  • A Battery Problem
  • A Bad Alternator
  • A Problem with the Car’s Starter Motor
  • An Issue with the Fuel Filter
  • A Bad Fuel Pump
  • Bad Fuel Injectors
  • A Bad Oxygen Sensor
  • Bad Spark Plugs
  • Problems with the Car’s Throttle Body
  • Something Obstructing Your Air Filter

1: Your Car Has a Battery Problem

Anytime I’ve had a car hesitate to start, my first thought is that it’s a battery problem. Batteries have a limited lifespan, and changes in the local weather can turn a minor battery problem into a major one, literally overnight. Battery problems can vary, so you need to pay attention to the symptoms and the severity of the problem.

There are a few things that can add to your suspicion that it’s a bad battery causing your car to hesitate to start. This includes things like:

  • The car is sluggish to start
  • The interior lights & console dim when cranking
  • The sound of turning over gets slower and slower
  • You hear a clicking noise at the end of a sluggish start

Is It Safe to Drive with a Bad Battery?

The good news is that if your battery is going bad, or there is some sort of connection problem between the battery and the rest of the vehicle’s electrical system, you can still safely drive it home or to a repair shop. Though once you get there, the car likely won’t start again.

The other problem you have to consider is that your car could have a bad alternator, and it’s not safe to drive with a bad alternator.

Since they both share a lot of similar symptoms, it’s best to test the battery first to know for sure your car isn’t going to end up dead in traffic with a bad alternator.

How to Test for a Bad Battery?

Determining if a bad battery is causing your car to hesitate to start begins with a visual inspection and testing with a voltmeter, load tester, or multimeter. 

Look for Corrosion on the Battery Terminals: Star by popping the hood and giving the battery a thorough visual inspection. Look at the terminals for signs of corrosion. If you see fuzzy white, gray, or green deposits around the positive red terminal it could be that corrosion is preventing the battery from fully delivering the cold cranking amps the car’s engine needs to start robustly.

Check for Loose Battery Connections: Also, check for any loose connections. If anything is loose on either terminal, or the ground wire for the car is loose it can also impede the battery’s ability to deliver efficient charge.

How to Test Your Car’s Battery

If everything looks in good order, the terminals are clean and the connections are all rock solid, then you need to test the voltage and/or load capacity of your car’s battery. You can do this with a voltmeter, multimeter, or load tester and the following steps.

  • Step One: Set your multimeter or voltmeter to read 15 to 20 volts.
  • Step Two: Touch the red node on the positive terminal first before touching the black node on the negative terminal.
  • Step Three: Check the reading on the display.
  • If the car is off and the battery reading is less than 12.6 Volts chances are good your car’s battery is dying. If the car is still running and you get a reading of 10 Volts or less, then it also means your battery is dying.

How to Test Your Car’s Battery If You Don’t Have a Multimeter or Voltmeter?

Most auto parts and battery stores offer free battery tests. If your car is running, you can just drive directly to:

  • Auto Zone
  • O’Reilly Auto Parts
  • NAPA Auto Parts
  • Batteries+
  • Walmart Auto Care Center

Will Auto Parts Stores Install a New Battery?

Since you most likely will need to buy a new battery, you’re likely going to end up at one of these stores. Though I would note that a store with a service center like Walmart Auto Care and Batteries+ will put a new battery in for you.

Whereas an auto parts store without a service center will test your battery and happily sell you a new one, you’ll have to install it yourself.

Once upon a time, I limped a truck with a bad battery to an auto parts store without a service center.

One of the connections was corroded so badly that it fused to the terminal. The entire wire needed to be replaced and I didn’t have the tools with me to do it in their parking lot in the middle of winter.

If you’re worried at all about your ability to make the battery swap yourself, and/or the weather outside is bad, then the wisest course of action is to get your battery tested and replaced at an auto parts store with a service center.

How to Install a New Battery

The easiest way to fix a bad battery at the end of its life is to replace it. This is assuming your connections are clean,the wires are in good repairand you have a basic automotive tool kit, and the following steps.

  • Step One: Park the car, turn it off, and set the parking brake.
  • Step Two: Disconnect the negative terminal first. Then disconnect the red positive terminal.
  • Step Three: Remove the clamp holding the battery in the engine bay.
  • Step Four: Remove the battery, and wipe the battery mounting plate clean.
  • Step Five: Set the new replacement car battery in place with the red and black terminals in the same orientation as the original battery. If you get this wrong the wires likely won’t be long enough to connect.
  • Step Six: Put the battery clamp back on and secure it firmly in place.
  • Step Seven: Put the washers in place, and apply some dielectric grease if you have some.
  • Step Eight: Connect the positive red terminal wire first, then the black negative terminal wire second.

2: Your Car Has a Bad Alternator

The next most common reason why a car hesitates to start is a failing or bad alternator. This is a special component inside the car that works in tandem with the engine to charge and maintain the charge in the battery. Your car can hesitate to start but still turn over off just the residual charge in the battery. Though without a properly functioning alternator, the amount of time your car will continue to run normally is finite.

Symptoms of a failing, or bad alternator can be very similar to a battery problem. This includes things like:

  • Dim Lights
  • The Battery Warning Light Turning On
  • A Dead Battery That Still Tests as Good
  • Malfunctioning Accessories
  • Slow Accessories
  • Trouble Starting with Frequent Stalling
  • Whining Noises
  • The Smell of Burning Wires

Is It Safe to Drive with a Bad Alternator?

Unless there is a good auto mechanic shop within just a few blocks, it’s a bad idea to try to drive a car with a failing or bad alternator. The car will live off the ambient charge in the battery, which can run out at any moment. Including in the middle of traffic!

Years ago, I was trying to nurse a failing alternator until I had the time and money to fix it properly. Then it finally died when I was out running errands. I got lucky and was able to coast into my driveway with a completely flat battery with so little charge that I couldn’t even roll up the power windows!

How to Test Your Car’s Alternator

Testing your car’s alternator is relatively simple. Though you’ll likely need another person to help you and a voltmeter or a multimeter. When you’re ready you can open the hood and locate the starter motor near the battery.

Have another person turn the ignition key to the “Start” position while you listen for the starter motor to engage in the engine bay.

If you hear a clicking or a grinding noise, but it doesn’t start or the car hesitates to start, it very likely is a problem with the starter motor solenoid. If you don’t hear anything at all, the problem is probably in the starter motor itself.

You can test the starter motor’s solenoid by bridging or jumping the circuit by connecting the terminals with a screwdriver or a conductive piece of metal. If the engine starts like normal while you’re bridging the terminals, then the problem is with the starter solenoid. If the problem persists, then the problem is with the starter motor itself.

You can then test the starter motor by disconnecting the wires from the solenoid. Then use a voltmeter or a multimeter set to test voltage to see if there is charge continuity across the terminals. If there is no connection, the car’s starter motor willneed to be rebuilt and/or replaced.

Is It Better to Replace or Rebuild an Alternator?

The easier fix is often to replace a bad alternator replaced rather than pay a mechanic to rebuild it.

Most of the time the cost of a new alternator is only slightly more than the labor cost to have a mechanic rebuild it. You also get the peace of mind that comes with knowing that everything is in factor-perfect condition with the replaced alternator.

The cost to have a mechanic replace your car’s alternator ranges from $225 to $400. Most of this is the cost of the mechanic’s labor time.

Can I Replace My Car’s Alternator Myself?

If you’re mechanically inclined and you have the right tools, you can save a lot of money by simply replacing your alternator yourself. This is something that a capable home mechanic can do in a single Saturday afternoon.

The cost for a new alternator from an auto parts store ranges from $125 to $250 depending on the model and the size of the alternator.

How to Replace Your Car’s Alternator

Once you’ve sourced the correct replacement alternator you can install it using the following steps.

  • Step One: Park the car in a level area and set the parking brake.
  • Step Two: Completely disconnect the battery and pull it out of the engine bay to give yourself room to work.
  • Step Three: Take a picture of the alternator on your phone, before carefully disconnecting the wires leading to the alternator. Then make sure to label them for later as well.
  • Step Four: Carefully remove the belt from the pulley. Be prepared to loosen the tensioner in the process.
  • Step Five: Remove the bolts holding the alternator in place. The number of bolts can vary by model, but often times there’s one in the front and two in the back.
  • Step Six: Pull the old alternator out and set it next to the new one to make sure that they are indeed a perfect match.
  • Step Seven: Reverse the steps you took to remove the old alternator, to install the new replacement alternator in its place.
  • Step Eight: Start the car and test to make sure that the new alternator is charging the battery.

3: There’s a Problem with Your Car’s Starter Motor

Your car’s starter motor plays a critical role in engaging the engine with vigor. It’s essentially a small gear with a collection of interlocking teeth. As soon as you turn your key or press the start button in the cab, the internal solenoid in the starter closes two contacts together this relays electrical currents from the ignition to the starter motor.

At the same time, the starter motor instantly engages the engine’s flywheel. This rotation delivers enough power to start the car’s engine rotating. This initial rotation also serves to draw air and fuel into the cylinders which ignite the engine’s internal combustion process. From there your car’s motor runs on its own.

Overtime the teeth on the starter motor will start to grind or degrade due to excess wear and tear. This can be accelerated by high temperatures in the engine bay. Especially if you have a bad habit of cranking the engine for long times, causing a grinding noise.

The teeth themselves can grind down over time and with heat. It’s also possible for an accidental engagement of the starter motor to break off one or two of the gear teeth. This can happen from something as simple as accidentally turning the key while the engine is running.

It’s also possible for the starter’s solenoid to degrade or burn out over time. If it fails then your car’s starter motor can’t get the electricity it needs to start the rotation necessary to start the internal combustion process.

Symptoms of a Bad Starter Motor

  • Car Hesitates to Start
  • Clicking, Grinding or Whirring Noises While Starting
  • The Starter Stays on After Starting& Grinds
  • Smoke Inside the Engine Bay
  • The Starter Engages but Whirrs & the Motor Won’t Start
  • A Buzzing Noise When You Turn the Key

Can I Drive a Car with a Bad Starter Motor?

So long as the car starts, a bad starter motor won’t leave you dead on the side of the road. The risk is that you get to your destination, turn the car off and then the starter can’t get the engine to turn over again.

I once had a starter motor with two bad gear teeth. Sometimes it would start perfectly fine, sometimes the car would hesitate to start. I pushed my luck for a while until it finally wouldn’t start and I had to call for a tow truck.

Is It Better to Rebuild or Replace a Bad Starter?

The cost to rebuild a starter is very similar to the cost to replace a starter. Though most home mechanics don’t have all the skills and tools to rebuild a starter themselves. So, if you’re going to try to do it yourself, it’s probably better to replace your car’s bad starter with a new one.

The cost to have a mechanic rebuild a car’s starter ranges from $150 to $650, with professional installation.

The cost of a new starter motor ranges from $80 to $350 before installation.

How to Replace Your Car’s Starter Motor

If you are mechanically inclined and you have the right tools, it might be possible for you to replace your bad starter with a new one using the following steps.

  • Step One: Shut the car off, set the parking brake, and give it time to cool down in the engine bay.
  • Step Two: Disconnect both wires leading to the battery terminals.
  • Step Three: Disconnect the large positive cable that runs from the battery to the starter motor at the starter.
  • Step Four: Use a ratchet set to disconnect the bolts holding the starter to the engine block.
  • Step Five: Make sure to also remove all other bracket supports that help hold the starter in place.
  • Step Six: Remove the starter motor from the engine bay of the car.
  • Step Seven: Compare the new starter with the old one to make sure they are a perfect match.
  • Step Eight: Reverse the steps to install the starter motor by mounting the new starter on the block and attaching the bolts as well as all other supporting brackets. Then reconnect the cable running from the battery to the starter.
  • Step Nine: Reconnect the battery and test the car to make sure it starts normally.

I think you are surprised by how vigorously it starts again. A clear realization that your car’s old starter motor had been going out for a while, but the decline was so slow, you just didn’t notice it until the car hesitated to start.

3: There’s a Problem with Your Car’s Fuel System

The fuel system in modern-day cars can be complex, yet there are some common components and fuel system problems that can cause your car to hesitate to start. This can include issues with the fuel filter, the fuel pump, bad fuel injectors, and even degraded low-quality fuel in the car’s gas tank.

If you’ve tested the car’s battery, alternator, and starter motor, the next most likely suspect causing your car to hesitate to start is a problem with the fuel system. This is even more likely to be the case if you have a bad habit of letting your car get down to the fuel warning light, or if it’s been sitting idle for two weeks or more.

Fixing Problems Low-Quality Separated Fuel

If your car has been sitting for more than two weeks without a fuel stabilizer in the tank, the gas can separate drawing moisture and other impurities into the fuel system.

The easiest fix here is simply to gas up with high-octane fuel and perhaps some injector cleaner. If you know the car is going to need to sit again, you should also consider adding the appropriate amount of fuel stabilizer.

Fixing a Clogged Fuel Filter or a Fuel Pump Problem

Lingering tank deposits can get pulled into the fuel system when you run the gas tank below a quarter. If this happens frequently, it can clog your car’s fuel filter, clog the fuel injectors, or other components inside the fuel system.

A clogged fuel filter can also put excess wear and tear on the fuel pump that delivers gas from the tank to your car’s engine. As time goes on, it can accelerate the inevitable death of the fuel pump.

Symptoms of a Bad Fuel Filter

  • The car hesitates to start
  • Idling roughly
  • Car stalls when idling for too long
  • Poor acceleration that gradually worsens
  • Occasional misfires

Is It Safe to Drive with a Bad Fuel Filter?

You might be able to get away with driving a car with a clogged fuel filter for days if not weeks. Though it will eventually reach the point where the car goes from hesitating to start and being gutless when accelerating, to not starting at all. Once this happens, you’re usually staring at replacing both the fuel filter as well as the fuel pump!

Can I Replace My Own Fuel Filter?

Ideally, you should have your car’s fuel filter replaced every 2 to 3 years or every 30,000 miles. Though it can be tricky in some fuel-injected cars, and there might be some model-specific things to keep in mind when it comes to depressurizing your car’s fuel system. Still, it’s something within reason for someone with a decent set of tools and mechanical sympathy.

How to Replace Your Car’s Fuel Filter

Once you’ve sourced the proper fuel filter for your car’s make and model, you can replace it via the following steps.

  • Step One: Find your car’s primary fuse box and remove the fuse or relay that connects power to your fuel pump. If you’re not sure which one is specifically for your fuel pump, you should be able to find it in the owner’s manual. This will depressurize the fuel system.
  • Step Two: Start your engine and let it run until it stalls. This will draw all the lingering fuel out of the system. (Don’t be alarmed if the engine warning light comes on momentarily.)
  • Step Three: Crank the engine again for 5 to 7 seconds to make sure all the fuel pressure is released from the car’s fuel lines. Then turn the ignition back to the OFF position.
  • Step Four: Disconnect the car battery’s black ground cable (With the – sign).
  • Step Five: Locate your fuel filter in the owner’s manual. Then lift your car with a stable jack and block it off to keep it safely suspended. Most of the time the fuel filter is in the rear of the car near the fuel tank.
  • Step Six: Place a drip pan under the fuel filter, and put on safety glasses and other protective clothing, as there might still be a few drips of fuel in the system. Keep some clean shop rags handy as well.
  • Step Seven: Find the fuel filter and use a screwdriver to detach the clips connecting it to the fuel line. Note: Some car models have release buttons that make this easier.
  • Step Eight: Loosen the clamp or band holding the fuel filter in place. It’s usually something similar to a large pipe clamp with a screw you can reverse.
  • Step Nine: Completely disconnect the fuel line fittings from the filter. Don’t be surprised if a little fuel leaks out with it.
  • Step Ten: Remove the fuel filter. Look to see if there’s an arrow on it indicating the direction of the fuel flows. Compare this to the new filter. This is important as not all replacement filters have this arrow.
  • Step Eleven: Install the new filter making sure that the fuel flow is in the correct orientation. The direction of fuel flow will always be toward the vehicle’s engine, which is often in the front.
  • Step Twelve: Use the clips that came with the replacement fuel filter to connect it to your car’s fuel line and tighten all the clamps brackets.
  • Step Thirteen: Reinsert the relay/fuse for the car’s fuel pump, and reconnect the battery’s ground wire to the black battery (-) terminal. Then lower your vehicle so it’s sitting on level ground normally.
  • Step Fourteen: Carefully turn the car’s key to the ON position, without actually starting it. Then turn it to the OFF position for 3 to 5 seconds and back to the ON position. This will draw pressure back into the fuel lines.
  • Step Fifteen: With the system properly pressurized again, you should check under your vehicle for any leaks. This is an important safety step that should not be skipped!
  • Step Sixteen: Start the engine normally, and let it run for 30 to 45 seconds checking under the car again for any leaks.

4: Your Car Has a Bad Fuel Pump

Fuel pumps have a long, yet finite lifespan. They can be more prone to premature failure if you have a bad habit of ignoring your fuel filter. If you found that you had a clogged fuel filter in your previous fixes, and your car still hesitates to start, then you might have a bad fuel pump.

Symptoms of a Bad Fuel Pump

  • A Loud Wining or Buzzing Sound Coming from Your Car’s Fuel Tank
  • The car hesitates to start
  • The Engine Sputters
  • Frequent Stalling
  • Feeling Down on Power
  • Surging Power
  • Poor Fuel Efficiency

Is It Safe to Drive with a Bad Fuel Pump

When a fuel pump is going out, driving on it will just affect the performance of the engine. It’s when the fuel pump finally dies that you will be left stranded, and you can never tell for sure when that will be.

Years ago I went on a camping trip with my friends who were knowingly driving around with a failing fuel pump. The last morning of the trip, I left at dawn to drive back home. When I got home I found out that their SUV didn’t start at all. They were completely stranded three hours from home and had to pay an expensive tow bill to get them out of the forest.

So, if your car is hesitating to start, and you strongly suspect it’s the fuel pump. I would recommend carpooling to work until you can get it fixed.

Can I Fix a Bad Fuel Pump Myself?

Fixing a fuel pump yourself is a risky proposition, that only an experienced home mechanics should consider doing. You’re dealing with one of the major components of the fuel system. Most fuel pumps are located in the gas tank. So there are serious flame and hazardous material concerns in the equation.

The average cost to have a mechanic replace your fuel pump ranges from $250 to as much as $1,000.

The cost of the replacement fuel pump itself ranges from $125 to $300 depending on the make and model of the vehicle.

5: Your Car Has Bad or Clogged Fuel Injectors

Most modern-day cars are fuel injected. Fuel injectors help to control the volume and timing that gasoline is introduced to the combustion chamber during the power stroke. If they are clogged by debris that enters the fuel system, or other deposits it can significantly hamper your car’s performance. Not the least which is causing your car to hesitate to start.

Inside each fuel injector, there is a small spring, solenoid, and a pin. When the fuel injector is receiving an electrical charge, it creates a minute magnetic field that pulls the pin up. This allows the pressure of the fuel line to spray gas into the engine. When the charge is gone, it closes the pin again, the spring automatically pushes the pin back down to ensure no more fuel is delivered until the next combustion process is needed.

Signs of Bad Fuel Injectors

The symptoms of bad fuel injectors can manifest gradually over time to the point where it can be hard to notice true performance issues until the problem is severe. Though sometimes deposits that make it through a bad fuel filter can go straight to one or more of the injectors causing an overnight problem.

If you’ve recently had fuel filter or fuel pump problems and you notice one or more of the following symptoms, then it might be bad fuel injectors causing your car to hesitate to start.

  • The Car Has an Increasingly Rough Idle
  • The Car Sputters or Shakes When Stopped in Traffic
  • The Engine Misfires
  • The Car Hesitates to Start
  • Increasingly Poor Fuel Economy
  • The Car’s RPM Needle Dances
  • The Check Engine Light Come On

Is It Safe to Drive with Bad Fuel Injectors?

Bad fuel injectors can affect the engine’s performance in various ways. Not the least of which is causing the engine to run lean. This can cause damage to moving parts which is very expensive to fix. If the engine is misfiring frequently, the unburned fuel can also badly damage the engine, not to mention severely compromising the catalytic converter. All of which are very expensive fixes.

If you have bad fuel injectors causing your car to hesitate to start, it might be okay to drive home, to the mechanic or the auto parts store. Still, I wouldn’t use the car as your daily driver again until the problem has been properly fixed.

How to Test for Clogged Fuel Injectors

The easiest way to determine a fuel injector problem is to hook the car up to a code reader. Bad injectors will throw a code of P0300 through P0308, depending on how many cylinders your car’s engine has. You can then further confirm your suspicions with a simple screwdriver test.

Fuel injectors are typically found on the top of the engine along the fuel rail on each side of the engine block. Once you’ve located them you need to place a long screwdriver at the back of the fuel injector, then put your ear near the handle. (This works better with plastic handles than wood or other cushioned handles).

A properly functioning fuel injector will make a consistent pulsing or clicking noise. If the fuel injector in question is quiet or is firing inconsistently is likely clogged or bad.

How to Test Fuel Injectors with a Multimeter

The code reader and/or the screwdriver test won’t tell you for sure if your car’s fuel injectors are clogged or bad. This calls for setting the multimeter to read Ohms.

  • Step One: Look up the Ohms resistance of your vehicle’s fuel injectors with a simple Google search or in your service manual.
  • Step Two: Turn your multimeter on and make sure it is set to read Ohms at 200.
  • Step Three: Depress the clip holding the wire to the fuel injector, this will reveal the electrical prongs on the fuel injector.
  • Step Four: Touch each fuel injector with the multimeter probes, testing each injector at a time.
  • Step Five: Check to make sure that each injector is within .5 Ohms of the normal specs for your make and model. Hold the probes in place for 2 to 3 seconds or until the reading levels out.

If one of the injectors is more than .5 Ohms off, then it likely needs to be cleaned or replaced.

How to Clean Clogged Fuel Injectors

Let’s say you’ve thoroughly tested your fuel injectors and you’ve come to the solid conclusion that your car hesitates to start due to clogged fuel injectors. Yet all your injectors are still giving you the proper Ohm reading for your make and model. You can then clean clogged injectors with some fuel system cleaner and the following steps.

  • Step One: Carefully disconnect your car’s negative battery terminal, and cover it or wrap it to avoid electric shocks.
  • Step Two: Remove the injector covers, and inspect the injectors. Each one should have a miniscule screen attached to it.
  • Step Three: Use the fuel system cleaner according to the directions to clean the screens.
  • Step Four: Reassemble everything via the reverse steps you used to access the fuel injectors, and reconnect the negative battery terminal.
  • Step Five:Start the car and check to make sure that the engine running properly.

How to Replace Fuel Injectors

Let’s say that your car’s fuel injectors are giving you an Ohm reading that is more than .5 off the specs, and it’s causing your car to hesitate to start. In a time like this, cleaning the fuel injectors likely won’t be enough to fix the problem and you’ll have to completely replace them.

To do this, you’ll need to make sure you have an extensive set of pliers, a comprehensive ratchet set, and a torque wrench. Not to mention the correct replacement fuel injectors for your make and model. You can then replace the fuel injectors using the following steps.

  • Step One: Carefully disconnect your car’s negative battery terminal, and cover it or wrap it to avoid electric shocks.
  • Step Two: Gently remove each fuel injector’s electrical connector.
  • Step Three: Carefully each of the bolts holding your car’s fuel injectors.
  • Step Four: Extract the old fuel injectors and compare them to the new replacement injectors to make sure that they’re a perfect match.
  • Step Five: Carefully insert the new fuel injectors one by one.
  • Step Six: Use a torque wrench to properly tighten the bolts holding the fuel injectors in place per the specs in your repair guide or owner’s manual.
  • Step Seven: Reconnect each electrical connector to its corresponding fuel injector.
  • Step Eight: Start the engine to make sure it’s running properly.

7: There’s a Problem with Your Car’s Oxygen Sensor

In modern-day fuel-injected cars, oxygen sensors play a critical role in helping to regulate the engine’s air-to-fuel ratio. Most cars have more than one oxygen sensor, and each plays a role in optimizing your car’s internal combustion process. If one or multiple oxygen sensors go bad, the imbalance in the air-to-fuel ratio can cause the car to hesitate to start. It can also affect performance when you accelerate.

Oxygen sensors have a finite lifespan. They are even more likely to have a problem if you are traveling and rapidly changing altitude. The car might idle fine but will struggle under load. In hot weather, a lot of cars with bad oxygen sensors will have problems with overheating.

Symptoms of a Car with a Bad Oxygen Sensor

Sometimes the symptoms of a car with a bad oxygen sensor as subtle, and sometimes they are profound. Often times the symptoms start out minor and you only notice them when the sensor has completely failed. If your car is exhibiting one or more of the following, you should suspect a bad oxygen sensor is at fault.

  • Car Hesitates to Start
  • Poor MPG
  • Poor Acceleration
  • A Sulfur or “Rotten Egg” Odor from Exhaust
  • Black Smoke in Exhaust
  • Misfiring
  • The Car Runs Hotter Than Usual
  • Car Overheats Easy in Hot Weather
  • A Flashing Check Engine Light

Is It Safe to Drive a Car with a Bad Oxygen Sensor?

When an oxygen sensor first starts to go out, the effect on the car is minimal. As it worsens your car’s engine not only performs worse but overheats easily. This can lead to other damage in the engine, which can lead to staggeringly expensive repairs. Though even a modest overheating incident can leave you stranded on the side of the road.

I once had a compromised oxygen sensor go out while we were on vacation in Wyoming. The car couldn’t compensate for the altitude on its own and overheated. Stranding us 60 miles outside of Cheyenne on a 90-degree day.

How to Fix a Bad Oxygen Sensor

The good news is that with the right parts and some basic tools it’s relatively easy to replace your car’s oxygen sensor. Though you will need to identify the correct sensor, which requires an OBD II Scan Tool. Though some auto parts stores will do this for you for free. You just have to call in advance.

From there you just make sure that you have the correct sensor for your car’s make and model year. You can then fix the oxygen sensor via the following steps.

  • Step One: Connect the OBD II scan tool to your car and read the codes to determine the specific oxygen sensor that’s failed.
  • Step Two: Lift the car with a jack and block it out to keep it safely suspended throughout the job.
  • Step Three: Disconnect the oxygen sensor from the wiring harness.
  • Step Four: Disconnect the oxygen sensor. You might need a ratchet/socket or an open-face box wrench to do this.
  • Step Five: Compare the new oxygen sensor to the old one to make absolutely certain you have the correct replacement part.
  • Step Six: Install the new oxygen sensor and reverse the steps you used to remove the old one.
  • Step Seven: Use the OBD II scan tool to clear the codes. If you skip this step the check engine light will remain on, and your car won’t be able to tell you if something else has gone wrong in the future.
  • Step Eight: Start the vehicle and take it for a short test drive. Make sure to check the acceleration, and if possible, test the MPG fuel consumption.

8: There’s a Problem With Your Spark Plugs

In a gasoline engine, sparkplugs play a critical role in igniting the vaporized fuel and air. As time goes on sparkplugs can suffer excess wear, tear, corrosion, and a buildup of carbon deposits. This can impede their performance enough to cause your car to hesitate to start.

You’re even more likely to have sparkplug problems if you have a bad habit of driving around with a dirty air filter, or if you do a lot of driving at low speeds. At the same time, a car that spends a lot of time idling to stay warm, or suffers from dirty fuel injectors can also contribute to the accelerated decline in sparkplug performance.

Common Symptoms of a Car Bad Spark Plugs

  • A car that hesitates to start
  • A Rough Idle
  • Noisy When Idling
  • The Engine Misfires
  • Increasing Misfiring Problems
  • Engine Surging
  • Poor Fuel Consumption
  • Poor Acceleration

Is It Safe to Drive with Bad Spark Plugs?

If you continue to drive for days or even weeks with bad spark plugs it can cause engine damage. The biggest risk here though is with misfires caused by bad spark plugs. When an engine misfires some of the unburned fuel remains. This can affect not only engine performance, but can potentially damage the catalytic converter which is an enormously expensive, and unnecessary repair.

If your car hesitates to start due to bad spark plugs you might be able to drive it for a day or two until you can get it fixed or get it into a shop. Any longer than that and you’re playing with fire and your bank account at the same time.

How to Test for Bad Spark Plugs

If you’ve been noticing symptoms of bad spark plugs, and you’ve already ruled out other issues such as battery, alternator, or fuel system issues, then the easiest way to test is to physically examine the spark plugs.

Though I would also recommend having the car hooked up to a code reader. Especially if you’ve had some misfiring issues. This can tell you which cylinders are misfiring as well as how many misfires you’ve had recently. Minor misfires can sometimes be hard to detect, but can still be a significant symptom of failing sparkplugs.

Usually, you can remove spark plugs with just a ratchet or a socket wrench. However, there are some older Ford vehicles, especially older Ford F-150s that are known for breaking the spark plugs when you try to remove them. This was originally Ford’s answer to coil problems back in the 2000s. They make a special “Spark Plug Removal Tool” to get their plugs out easily.

If you have a Ford that was made between 2004 to 2008, I would highly recommend investing in one of these spark plug removal tools, to use over a ratchet or any other tool.

Once you have your tool situation sorted out, physically inspecting spark plugs calls for:

  • Step One: Park the car on a level surface, set the parking brake, and let the engine cool down.
  • Step Two: Find the spark plugs on the engine block near the cylinders. 
  • Step Three: Disconnect the negative terminal of the car’s battery. Wrap or cover it to ensure no incidental contact with other metal components or the battery’s positive terminal while you’re working.
  • Step Four: Remove anything that might get in your way of easily accessing the spark plugs during the project.
  • Step Five: Carefully remove the spark plug wire, making sure not to tear the rubber boot or damage the wire housing. As you pull the rubber boot away, make sure it takes the metal terminal inside with it.
  • Step Six: Remove the ignition coil by disconnecting the electrical connector. This usually requires you to press down or pull up on the locking tab to release the coil. You might also need to remove the hold-down bolt. Then gently rotate the coil about a quarter turn.
  • Step Seven: Pull the coil straight out.
  • Step Eight: Remove each of the questionable spark plugs. Inspect it and the state of the spark plug well for signs of corrosion, or damage.

Sometimes, corroded, dirty, or fouled spark plugs can be cleaned and reused, but in my experience, you’re just borrowing time. The spark plugs will inevitably have problems again in the short term and you’ll be back here again a few weeks or months later doing the same thing and replacing them.

When you consider that a single spark plug only costs between $10 to $25, I think it’s easier to just replace any fouled plugs, as long as you’re already crawling around the engine bay.

How to Replace Bad Spark Plugs

Right off the bat, let me say that you don’t absolutely need to use a torque wrench to replace spark plugs, but having one is still a good idea. It’s a $50 to $75 investment to get a modest-quality one. If you’re a home mechanic you will likely have it for the rest of your life.

When you’re ready, you can follow all the steps to remove a spark plug that we discussed earlier and then replace any bad spark plugs in your car via the following steps.

  • Step One: Check to make sure that you do indeed have the correct spark plug, and visually compare the suspected bad spark plug with the new one. (This is a photographic moment for future reference).
  • Step Two: If the spark plug isn’t pre-gapped, you might need to check it with a gapping tool to make sure it matches the manufacturer’s specs in the repair guide.
  • Step Three: Gently screw in each new spark plug. Then tighten it to the required specifications in the owner’s manual or repair guide. This is where that torque wrench proves itself a worthy investment!
  • Step Four: Reinstall the coil and the spark plug wires by reversing the previous steps from when you removed them.
  • Step Five: Reconnect the negative battery terminal.
  • Step Six: Start the car and check the engine performance. Watch for any signs of a check engine light. If the check engine light does come on or flashes, check again with a code reader to see where the problem lies.

9: Your Car Has a Dirty Throttle Body

Each car has one throttle body that helps physically regulate the airflow going into the engine. If you’ve eliminated an issue with an oxygen sensor, then the next culprit to look for is a dirty throttle body. The throttle body is usually located between the air cleaner or air filter and the engine’s intake manifold.

Throttle body problems are more common on older cars. Especially if you’ve been using low-quality fuel. As time goes on a problem known as “Coking” allows filth and grime to buildup in the throttle body, affecting its performance.

Common Symptoms of a Car with a Throttle Body Problem

  • The car hesitates to start
  • Feeling Down on Power
  • Struggling to Accelerate
  • Higher or Lower Idling Than Normal
  • Poor Gas Mileage
  • Check Engine Light

How to Fix a Dirty Throttle Body

If you opened up your throttle body to find significant coking problems gumming up performance, then cleaning it is pretty easy. Though you’ll need to go to an auto parts store to get some specific throttle body cleaner. When you’re ready, you can clean your car’s throttle body with the following steps.

  • Step One: Park the car, set the parking brake, and let the engine cool down completely.
  • Step Two: Remove the throttle body from the car via the instructions in the owner’s manual or repair guide specific to your make and model.
  • Step Three: Read the instructions on the cleaner. Make sure to give it the specified amount of time before wiping it away.
  • Step Four: Reinstall the throttle body on the car.
  • Step Five: Start the car and let it idle until warm. Then take it for a test drive.

10: Your Car Has a Severely Clogged Air Filter

The internal combustion process relies on the proper mixture of fuel and air. Your car’s air filter ensures that air can get to the throttle body and into the combustion chamber without bringing in particulate matter and other debris, which can foul the combustion process.

Most of the time an old or badly clogged air filter won’t affect the process so much that it completely inhibits combustion. Though a badly clogged air filter or debris such as a previously unknown mouse nest in front of the air filter can clog the system so badly that your car hesitates to start due to an insufficient fuel-to-air ratio.

I once had a time when my truck was hesitating to start, and after spending hours trying to diagnose what it was, I found out that a mouse had simply found a way into my air intake and had made a massive nest in front of my air filter. I could have saved a lot of time, had I taken 30 seconds to check the air filter first.

Is It Safe to Drive Without an Air Filter?

Let’s say you find yourself in a situation like I did, where there is something blocking your air filter and it is badly clogged. In a time like this, it’s best to manually clean your air filter as much as you possibly can. This includes vacuuming it to remove any loose particulate matter. At that point, you can put the air filter back in, and drive directly to an auto parts store to get a replacement.

Most replacement air filters only cost $20 to $45. This is far cheaper than the potential damage that could be done to your engine by driving without an air filter and risking debris getting into your engine’s combustion chamber.

Frequently Asked Questions

Why does my car start in the morning but not in the afternoon?

A car that starts in the morning but not in the afternoon likely has some sort of issue with the fuel system. Most likely the fuel filter is significantly clogged, and/or the fuel pump is compromised because of it. If the afternoon weather is very hot, it could also be an issue with your battery. To know for sure, you’ll have to have the fuel pressure tested and the battery tested.

Why does my car not start after sitting overnight?

Several things can cause a car to hesitate to start after sitting overnight or cause it to not start at all after sitting for several hours. Though the most likely culprit here is probably a dying battery. If this is a recurring problem, and it’s getting slightly worse every day or two, it’s likely the battery is on its last legs. If it’s coming on quickly, then I would strongly suspect it’s the alternator that is rapidly dying, which is delivering an increasingly diminished charge to the battery.

If my oxygen sensor is bad in the mountains will it act normally when I drive back down?

Most of the time, a dramatic change in altitude simply accelerates the death of a failing oxygen sensor. Even when you get back to a lower elevation, things might be a little bit better, but it’s still wise to replace the oxygen sensor as soon as possible. Especially if the weather is hot and the bad sensor has been causing the car to overheat.

Why does my car hesitate to start in cold weather?

A car that hesitates to start most likely has a dying battery and/or a bad alternator. You’d be surprised how a sudden late summer or early fall cold snap can reveal a battery that’s been slowly dying during the warm days of summer.

What Is the Average Lifespan of a Fuel Pump?

The average fuel pump has a lifespan of 75,000 to 100,000 miles. Though this can be dramatically shortened by frequently using poor-quality fuel, failing to replace the fuel filter every 45,000 miles, and running your fuel tank dangerously low before filling up.

How Often Should Fuel Injectors Be Cleaned?

The prevailing wisdom is that a car’s fuel injectors should be cleaned every 45,000 miles. If it’s been more than that, then you should strongly consider cleaning your car’s fuel injectors. Even if you suspect that there is some other problem causing your car to hesitate to start.


There are a lot of things that can cause your car to hesitate to start, but battery problems, an alternator issue, and a bad starter motor sit at the top of the list. If you have a bad habit of running your car with the tank near empty, or you are far beyond 30,000 miles since the last fuel filter change, you could also suspect a problem in the fuel system.

Taking the time to systematically troubleshoot the underlying cause, will make it easier to fix the reason why your car hesitates to start. If you suspect a bad battery, you can usually get it tested for free at auto parts or battery stores. If the battery light is on in the car’s console, but the battery tests good, then it’s likely an alternator issue. Driving at this point is a risk, as time is running out.

If the battery and alternator are both good, then the next thing to look into is the starter motor or the fuel filter. Though increasing problems with acceleration and a poor mile-to-the-gallon rating might steer you instead to suspect one or more of your oxygen sensors has gone bad. Especially if you’ve recently been traveling at a higher elevation.

If you suspect the problem is linked to your spark plugs or fuel injectors, you’ll need to take a deeper dive into the engine bay. Make sure you have a multimeter in hand to test the performance of the fuel injectors. It often helps to test the injectors and inspect the spark plugs at the same time. This will help you eliminate one over the other.

Jason Farrell

Written By

Jason Farrell

Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.

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