8 Warning Symptoms Of A Bad Throttle Body

The throttle body of a car plays a critical role in delivering the necessary air to the combustion chamber to support vigorous internal combustion. While it’s engineered to have a long life, time and coking deposits can start to take a toll on a throttle body, causing the car to feel down on power, have poor gas mileage, and have trouble accelerating. Just to name a few.

If you suspect your car has a clogged up or malfunctioning throttle body, you need to dial in the symptoms before considering whether you should clean, repair or replace it. dirty, clogged up or malfunctioning

The longer you drive a car that’s showing dirty or bad throttle body symptoms, the more likely you are to suffer problems with other important engine components.

8 Symptoms of a Bad or Failing Throttle Body On A Car

The early symptoms of a failing or bad throttle body sometimes start out minor. Though left unchecked, they become more and more pronounced. The progression of these symptoms typically gets worse based on the internal state of the throttle body and any other issues that might compound the imbalance in the fuel/air ratio.

1. An Increasingly Rough Idle

A Rough Idle

One of the earliest symptoms of a bad throttle body is a rough idle. This is a time when the throttle body and the idle airflow control valve are working together.

Grime, carbon deposits caused by coking, or other faults with the throttle body end up having a more profound effect on the ECU’s ability to maintain proper airflow to the combustion chamber at idle.

2. The Car Feels Down on Power

The Car Feels Down on Power

Due to coking and carbon deposits, throttle body problems often limit the airflow to the combustion chamber. This imbalance in the fuel/air mixture often leaves the car feeling increasingly down on power.

3. Poor or inconsistent Acceleration

Poor or inconsistent Acceleration

Coking deposits and physical faults in the throttle body can also start to affect responsiveness. You tend to notice this most when trying to accelerate rapidly.

You might try to merge up to highway traffic one time, feeling like you desperately won’t be going fast enough by the time you reach the stream of other vehicles. Then the next time you need to step on it to merge, the car suddenly responds normally.

This sort of inconsistency can lull you into thinking the problem is a short-term fault in the air or fuel system. Though it’s really that the throttle body is starting to fail, and the acceleration problem is doomed to recur more often and with greater severity as time goes on.

4. Poor Gas Mileage

Poor Gas Mileage

The inconsistent imbalance in the fuel/air mixture causes inefficient combustion. As time goes on, the lack of sufficient airflow leaves unburned fuel behind, which starts to affect fuel consumption.

This is a problem that tends to start out slight at first and then progressively gets worst quickly in lock-step with acceleration issues.

5. High RPMs at Idle

High RPMs at Idle

As the fickle relationship between the bad throttle body and the idle air control valve worsens, it can cause the engine to idle at higher-than-average RPMs.

A lot of times, this problem is compounded by the ECU failing to compensate correctly for the imbalance in the fuel/air mixture.

Depending on the tolerances programmed into your car’s ECU, this is usually the point right before you see a check engine light on the dash.

6. Stalling at Idle

Stalling at Idle

A car that goes from high RPMs and an increasingly rough idle to straight-up stalling at idle has a bad throttle body that has a high risk of causing damage to other engine components. Not to mention the very real risk of accidentally getting stranded somewhere.

7. Misfiring


Misfiring caused by a bad throttle body is due to the fuel/air imbalance allowing unburned fuel to linger in the combustion chamber.

It then ignites out of the cycle and/or can pass into the exhaust system, where it can do significant damage to the catalytic converter and other exhaust system components.

8. The Check Engine Light Come On

The Check Engine Light Come On

There’s nothing about a check engine light coming on that specifically screams “Bad Throttle Body.” Though it’s certainly a sign that the ECU has had enough, and you can start to guess what the problem is by the other symptoms.

What the check engine light is also telling you is that it’s time to hook up an OBD II code reader.

If it’s throwing a code P0121 indicating a problem with the throttle position sensor, it means you have a failing or bad throttle body that needs to be addressed as soon as possible.

What Causes Throttle Body Problems?

Throttle body problems are typically caused by the buildup of grime, dust, and carbon deposits created by a process known as coking.

As they continue to accumulate, these deposits affect the performance of the butterfly valve that the throttle body uses to regulate the airflow going into the combustion chamber precisely.

This incremental nature of this coking process is the main reason why throttle body issues start out minor before eventually accelerating into more serious issues such as dangerous misfires and stalling out at idle.

Should I Replace My Car’s Throttle Body or Clean It?

Should I Replace My Car’s Throttle Body or Clean It

If you catch a dirty throttle body problem in the early stages, you might be able to clean it to restore it to new operating conditions simply. But If your car’s throttle body is severely compromised, succumbing to throttle bore wear or severely gummed up with coking deposits to the point that even with proper lubrication, the blades refuse to turn smoothly, then you might have to replace it completely.

How To Properly Clean A Throttle Body

You can then clean the throttle body using the following steps, and all you need is some throttle body cleaner, Screwdrivers, Torx bits, or an assortment of socket wrenches.

  • Step One: Disconnect the negative terminal on the car’s battery as a safety precaution and put on some eye protection.
  • Step Two: Locate the throttle body and take a few detailed pictures of all the various hoses. When reassembling, you should also label them with a permanent marker and some masking tape for future reference.
  • Step Three: Carefully remove the air duct connected to the bad throttle body without accidentally disconnecting any wires.
  • Step Four: Expose the throttle body. If you have a drive-by-wire throttle, make sure not to directly alter any connection to it or the traction control system.
  • Step Five: Spray the throttle body cleaner and use it according to the instructions on the can. This might require waiting a few minutes before lightly scrubbing the black deposits from the throttle body.
  • Step Six: Use clean paper towels or shop rags to wipe the throttle body clean thoroughly. Then visually inspect it.
  • Step Seven: You might need to perform multiple spray cleaning sessions to completely remove all the coking deposits from the throttle body.
  • Step Eight: Put a tiny drop of household general-purpose oil on the shafts of the throttle body to help the blades move fluidly.
  • Step Nine: Reverse the steps to reassemble the throttle body and the air duct and reconnect any hoses. Then reconnect the battery.
  • Step Ten: If your code reader threw a P0121 code, make sure to clear it.
  • Step Eleven: Start the car and let it idle for three to five minutes before taking it on a test drive.

How Much Does A Throttle Body Replacement Cost? 

Throttle Body Replacement Cost

The part cost for just a new replacement throttle body ranges between $225 to $550. There’s not a lot of labor that goes into having a mechanic do the replacement. So, you’re probably only looking at another $50 to $150.

The final cost to have a mechanic replace your car’s throttle body will run between $275 to $700, with parts and labor included.

Can I Replace the Throttle Body Myself?

Faulty ETB? | How to replace a Throttle Body FAST and EASY yourself

Replacing a car’s throttle body is something that a lot of home mechanics have done. Just take your time labeling all the hoses and parts, as well as taking tons of pictures. I was lax on this step once and it ended up adding an extra hour or so to the job.

Once you’ve sourced the right parts and assembled your tools, you can replace your car’s bad throttle body via the following steps.

  • Step One: Disconnect the negative terminal on the car’s battery and don some safety glasses as a safety precaution.
  • Step Two: Take detailed pictures and label all the various hoses.
  • Step Three: Carefully remove the air duct connected to the bad throttle body.
  • Step Four: Carefully disconnect the throttle cable from the throttle body control lever. If there’s any risk of slipping away, secure it out of the way.
  • Step Five: Remove all of the air or vacuum hoses from the throttle body. Inspect each hose for signs of cracking, rubber rot, punctures, or loose clamps. If you see a problem, you will need to fix it as part of the throttle body repair.
  • Step Six: Reverse the throttle body’s mounting screws and gently pull out the throttle body assembly. Inspect the old gasket before discarding it.
  • Step Seven: Install the new gasket in its place.
  • Step Eight: Visually compare the new throttle body to the bad throttle body to make sure they are an exact match.
  • Step Nine: Install the new throttle body and reverse the disassembly steps.
  • Step Ten: Make sure any old code P0121 is cleared.
  • Step Eleven: Start the car, and let it idle for 5 minutes before taking a test drive

Frequently Asked Questions

How Long Does a Throttle Body Last?

Assuming you’re properly maintaining your vehicle, a throttle body should last between 75,000 to 100,000 miles. I make it a point to either have the throttle body inspected, cleaned, and replaced as part of every tune-up. 

Can You Drive with a Bad Throttle Body?

Early on, you can drive with a bad throttle body and not notice it. Though as the idle gets rougher and the MPG rating gets worse, driving with a bad throttle body becomes more and more of a bad idea. If your car starts misfiring, stalls at idle, or the check engine light comes on; you absolutely should not drive with a bad throttle body until you can affect a serious clean or a total replacement.

What Happens If You Drive with a Bad Throttle Body?

As coking deposits start to accumulate inside a throttle body, the only major risks are poor fuel consumption and the consequence of a hard idle. It’s when the idle gets really bad; the car starts to misfire or stalls at idle that you are in serious danger of causing major damage to the engine and the exhaust system.

These are typically signs of unburned fuel remaining in the combustion chamber due to a major imbalance in the fuel/air ratio. This unburned fuel can cause major misfires, which can damage pistons, cylinders, and valves, as well as other sensitive engine components.

In short order, the unburned fuel will make its way into the exhaust system, where it can wreak further havoc. Not the least of which causes severe and expensive damage to the catalytic converter. This will cause you to fail any emissions tests, compounding the cost of what is essentially a relatively cheap and easy cleaning or replacement job.


Bad throttle body symptoms can certainly creep up on you over time. Naturally occurring coking deposits are a real threat by the time the car reaches 65,000 to 75,000 miles. This usually manifests as a slightly rough idle and less than stellar fuel consumption.

As the hard idle gets worse, followed by poor acceleration and the car being down on power, it’s wise to take a proactive step toward inspecting the throttle body. Giving it a good cleaning and lubricating the shaft on the butterfly valves at this point can save you the cost of replacing a truly bad throttle body later.

If you get to the point where the car is stalling at idle, misfiring or the check engine light comes on, you need to stop driving the car until you get the problem fixed. The cost to replace a throttle body is mostly in parts.

You can replace it yourself at the cost of around $250 and a Saturday afternoon. Though considering the relatively low labor cost to have a mechanic do it, there’s certainly no shame in bringing it in if you’re short on time.

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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