Engine Coolant Temperature Warning Light

We’ve all been there: cruising down the highway, belting out our favorite tune, when suddenly – an unexpected thermometer or simply the word “TEMP” or a more alarming ‘ENGINE OVERHEATING’ illuminates on the dashboard. It’s the dreaded engine temperature light, casting a glow that’s as chilling as it is irritating. But what exactly does this unsolicited red light mean, and more importantly, how should you respond?

Internal combustion engines, the powerhouse behind most modern vehicles, create a huge amount of heat. Managing this heat is the job of your vehicle’s cooling system, which uses a carefully concocted mixture of anti-freeze and water to absorb the engine’s heat before returning it to the radiator for dissipation.

So, when that little coolant temperature warning light pops on your dash while you’re driving, it’s essentially your car’s way of saying, “Hey, there’s something off with my cooling process, and I’m getting a bit too hot under the hood.” Continuing to drive while your coolant temperature warning light is on could lead to severe consequences, including potential engine overheating, damage to vital components, and, in extreme cases, a complete engine seizure.

In this post, we’ll dig deeper into the common reasons why your engine temperature light might come on and, more crucially, what steps you should take when it does.

Why the Engine Coolant Temperature Warning Matters

A lot of modern cars have an engine coolant temperature sensor that actively monitors the temperature of the coolant in the radiator. This information is then sent to the car’s computer to help it adjust the timing and other conditions needed for peak internal combustion performance.

Most automakers set a threshold for the safe operating temperature of the engine based on the temperature of the coolant that passes through the engine block. It’s usually set around 240 to 250 degrees, which is just below the point where the moving components inside the engine start to be at risk of overheating damage

When the temperature of the coolant gets over that threshold, the car’s computer will activate the engine coolant temperature warning. In some cars, this is a simple red light shaped like a thermometer floating in waves. A warning message might come up in many newer cars with a digital dash display. Though in a lot of older vehicles, you’ll just get a curious check engine light as the engine temperature gauge inches closer to the red line.

When the check engine light comes on, it will throw one of the following codes, which you can pick up on a code reader. 

  • Code P0217 is a blanket code for Engine Over Temperature
  • Code P0118 indicates a problem with the engine coolant temperature sensor or its circuit.
  • Code P0128 indicates a problem with the thermostat.

7 Reasons Your Engine Temperature Warning Light Stays Illuminated

Most of the time, when a coolant temperature warning light comes on, it’s due to your coolant level being too low or something clogging the cooling system, preventing it from circulating and dissipating heat. It’s also possible for a stuck-closed thermostat or degraded coolant to hinder the engine’s ability to lose heat via the radiator.

Some of these things you can check and fix on your own. Some need the attention of a professional mechanic. Complicating things is that the engine will be rocket-hot, so it’s nearly impossible to check some possible faults without risking severe burns.

1. Low Coolant

Low Coolant

Low coolant is one of the most common reasons why an engine temperature warning light stays on or comes on while driving. The concern here is that the engine and cooling system don’t burn or use coolant over time. It’s a semi-sealed system that, when functioning properly, suffers virtually no evaporation. So, if you have low coolant, it means you have a leak somewhere that will need to be fixed before you can safely get on the road again.

Take a quick look under the car to see if you notice any obvious leaks. If the leak just started, it might be gushing with pressurized, hot coolant. If it’s been going on for a while, or it’s a slow leak that’s gradually lowered your coolant levels, then the leak might not be obvious. The coolant is likely still very hot, so this is more of a visual inspection. You don’t want to go feeling around for hidden wet spots with your hands until the engine block has completely cooled down.

Common Places to Look for a Coolant Leak

  • Radiator hose connection
  • The thermostat on the top of the radiator
  • Possible puncture in the radiator
  • A coolant line running from the engine to the radiator
  • The water pump
  • The head gasket

The Quick Fix for Low Coolant

If you find signs of a minor coolant leak and you need to make it home or to your mechanic’s garage, you can refill the coolant reservoir. Though you’ll need to let the engine cool all the way down first.

  • Step One: Once the engine is completely cool, take the cap off the coolant reservoir and let any ambient pressure equalize.
  • Step Two: Pour in just enough coolant to bring the line on the reservoir up to the COLD mark.
  • Step Three: Start the engine and let it run for a minute or two. Keep an eye on the coolant level, and look out for obvious coolant leaks under the car.
  • Step Four: If the coolant level starts to dip slowly, with the circulation through the system, add a little more in.

At this point, you can drive for a short distance to get home or to the mechanic. Just keep an eye on the temperature gauge and be prepared to pull over if the coolant temperature warning light comes on again.

2. Bad Coolant

Bad Coolant

Engine coolant that’s started to degrade or is fouled by contaminants does a poor job of cooling the engine, which will eventually cause the engine temperature warning light to come on. The average engine coolant lifespan is around two years or 30,000 miles. As it degrades, the fluid’s color will start to darken. You might even notice particles or sludge in the reservoir.

The other concern you have with bad or contaminated coolant is that it can eventually start to clog up your coolant lines or even cause a serious clog in the radiator itself.

How to Fix Bad Coolant

If your coolant temperature warning light comes on and your coolant is dark and sludgy, the first thing to do is perform a radiator flush. If you’re a modestly capable DIY mechanic, you can probably handle this on your own. All it takes is some distilled water, a radiator flush/cleaning solution, a drain bucket/pan, and some basic tools.

If this is beyond your depth, you can take your car anywhere that performs oil changes, and they can perform the flush and fill for around $75 to $125.

3. A Blockage in the Radiator or Coolant Lines

A Blockage in the Radiator or Coolant Lines

Debris in the radiator or the coolant lines running from the engine to the radiator can hamper coolant circulation, causing the heat to build up to the point that the engine coolant temperature sensor comes on. One of the early signs of a problem like this is the heater running increasingly cold as the circulation of the fluid worsens.

You might also notice strange-looking sludge in the coolant in the reservoir. If the system gets too hot, it can cause hose clamps and gaskets to fail, causing multiple little coolant leaks all over the engine bay. You might even notice a collapsed hose when the engine finally cools down, as the blockage prevents the system’s pressure from equalizing properly.

One of the easiest ways to confirm a suspected blockage in the radiator is to trace the heat with an infrared thermometer. Point the red dot on the intake line and follow it through the various runs and turns of the radiator.

If the radiator is clear, the temperature should start out hot, around 225 degrees, and cool gradually by the time it reaches the return line to the engine.

If the radiator is clogged, the temperature by the intake might be as hot as 240 degrees or more. Then you’ll have a point where you’re tracing the flow, and suddenly, the temperature drops significantly. This is the place where the clog is in the radiator. Since little to no fluid can get past the obstruction, the heat energy can’t transfer through the rest of the radiator.

How to Fix a Clogged Radiator

If a modest clog in the radiator is causing the engine coolant temperature warning light to come on, you might be able to clear it by performing a complete coolant flush.

If you’re uncomfortable handling the radiator flush on your own, or your first attempt to flush the radiator didn’t clear the obstruction, a mechanic can perform a professional radiator flush and fill for around $75 to $125.

In the case of a severe clog, you might need to have multiple leak fixes throughout the cooling system. The severe heat often breaks down small gaskets like the one that marries the thermostat to the top of the radiator.

4. A Stuck Closed Thermostat

A Stuck Closed Thermostat

Suppose the thermostat at the top of the radiator gets stuck closed. In that case, the coolant won’t be able to circulate properly through the system, causing the coolant temperature warning light to come on eventually. If you look at the top of the radiator, you might even see coolant leaking out as the thermostat gasket fails. At the same time, residue on the top of the radiator where the thermostat marries to the radiator is also a sign that the coolant has been super-heated to the point where it escapes around the failing gasket.

Another strong sign of a stuck-closed thermostat is a lack of cab heat. If you turn the interior heater on and the fan just blows cold air, it’s a sign that the engine coolant isn’t circulating. This traps heat energy in the engine block without letting it escape via the radiator or the heater core.

How to Fix a Stuck Closed Thermostat

There isn’t a good way to get a stuck-closed thermostat. Instead, you have to replace it. If you’re a reasonably capable DIY mechanic, you should be able to handle this yourself, which will save you about $45 to $60 in mechanic labor.

The other great thing about a stuck-closed thermostat is that the replacement part is relatively cheap. An auto parts store will probably only charge you $25 to $50 for a new thermostat. You should be able to replace it yourself in less than an hour.

5. A Bad Water Pump

A Bad Water Pump

If your car’s water pump goes out, it won’t be able to circulate the coolant, causing the temperature to rise until the coolant temperature warning light comes on. The water pump is pulley-driven by the serpentine belt. If the serpentine belt has been slipping, making whining noises, it’s frayed, or completely broken, the water pump won’t be able to circulate fluid. You would likely also have problems with the alternator, air conditioning, and all the other components the serpentine belt powers in the engine bay.

The water pump is also connected to the timing chain. Sometimes, the timing chain can go out, taking the water pump. In a scenario like this, the car usually makes a strange rattling noise when idling, braking, or starting. You might have even had occasional misfires right before the engine temperature warning light came on.

The heater core will run cold since the coolant can’t circulate without the water pump. However, this could be a symptom of other things, including a stuck-closed thermostat or a blockage in the radiator itself.

How to Fix a Bad Water Pump

If a bad water pump caused your coolant temperature warning light to come on, it must be replaced. This usually involves removing the timing chain, which adds extra labor costs. The wise move is often to have the timing chain replaced as well. Unfortunately, this job is usually beyond what a DIY mechanic can handle.

The part cost for a new water pump can be as low as $75 to as much as $200.

Chances are good your mechanic will correctly recommend replacing the timing chain, which will add another $75 to $155 to the cost of the parts.

It makes sense to have them replace the old timing chain since they’ll likely have to take it out to replace the water pump, so the additional labor cost is next to nothing.

The labor cost to have a mechanic replace the water pump and timing chain can vary wildly, but you can expect a real-world average of an extra $150 to $200.

So, to get the bad water pump and timing chain replaced, be prepared for a final repair bill of $300 to $550.

6. A Bad Head Gasket

A Bad Head Gasket

Even a small failure in the head gasket can allow coolant and oil to mix, increasing the friction and hampering the cooling system’s performance until the coolant temperature warning light comes on and stays on. This can also cause a low coolant level, as coolant can enter the combustion chamber, which passes through the exhaust manifold.

In such a case, you’ll usually see a big plume of white smoke coming out of the tailpipe when you start the car or accelerate hard. If the head gasket is severely blown or the cylinder head is cracked, you might notice white, sweet-smelling steam coming out of the tailpipe, and the engine will occasionally misfire. If you check the oil, which looks frothy or like chocolate milk, it means coolant will likely also end up in the oil pan.

How to Fix a Bad Head Gasket

You can use sealant additives to patch a minor leak in a head gasket temporarily. Most require you to flush the coolant and then refill it with water. Add the sealant and let the car run for a while to circulate the sealant until it adheres to the point of failure on the head gasket. Then, you flush the system and refill it with fresh coolant.

Sealant manufacturers love to tout their products as being long-term or permanent fixes. They might even hide a disclaimer somewhere that you just need to use more if it fails.

The truth is that these sealant additives tend only to be effective for minor leaks in a head gasket. They won’t do much to a blown head gasket. There’s also a chance that lingering deposits in the coolant system will cause clogs in the radiator or coolant lines in the future.

Even if everything goes off without a hitch, the real-world average lifespan of a head gasket sealant is usually around 3 to 6 months. In cold seasons, you might get six months. In the heat of summer, the sealant might not even make it three months.

Ultimately, these sealant products are a quick fix that will buy you some time to save up money for the much more serious head gasket replacement repair bill.

When it comes time to replace the head gasket, the parts cost itself is minimal. You can usually find a replacement gasket for $35 to $75 with the most popular models.

The problem is that to replace the bad head gasket, you need to take the top of the engine off, including the cylinder head. This is labor-intensive and usually beyond what a DIY mechanic can handle.

You can expect the labor cost to have a mechanic replace a blown head gasket to be around $375 to $550 or more.

However, the part and labor cost could go up even further if the cylinder head is damaged or there’s a cracked cylinder in the engine block.

7. A Bad Engine Coolant Temperature Sensor

A Bad Engine Coolant Temperature Sensor

In some cars, the coolant temperature warning light will start to flash if the ECU loses contact with the engine coolant temperature sensor. Just like any sensor it can burn out over time. It’s a thermistor, and past incidents where the engine ran hot could also lead to premature death.

A bad ECT sensor will also affect engine performance as the ECU won’t be able to get the engine timing and fuel/air mixture just right. You might notice dark smoke from the tailpipe, and your MPG will take a massive nosedive.

You can confirm that it’s the sensor by hooking up a code reader. The car’s computer will then throw a Code P0115 through P0119, indicating a problem with the engine coolant temperature sensor or its circuit.

It’s also possible for the sensor to die without the engine itself being overheated. So if the light starts blinking, take a moment to look at the temperature gauge. Then, pull over and see if the engine feels overly hot.

How to Fix a Bad Engine Temperature Sensor

Replacing the engine coolant temperature sensor is relatively easy and is within what an average DIY mechanic can handle. Most auto parts stores carry replacement parts for most makes and models.

The part cost for a new engine temperature sensor will only run you around $40 to $85. However, some exotic foreign models might be more. If you take it to a mechanic, you can expect them to add another $50 to $100 in labor costs.

Replacing the bad ECT sensor calls for partially draining out the coolant. If it’s been more than 30,000 miles, you might want also to perform a complete radiator flush and fill as long as you’re there.

You should be able to replace a bad ECT sensor yourself with some basic tools, a torque wrench, and the following steps.

  • Step One: Let the engine cool all the way down. Then disconnect the battery starting by taking off the black (Negative) lead first.
  • Step Two: Take a reference picture of the top of the engine with the plastic engine cover in place. Then remove it, making sure not to dislodge any hoses or wires.
  • Step Three: Locate the engine coolant temperature sensor. It’s usually near the thermostat but could be anywhere along the coolant lines or radiator if you’re not sure where yours is, check the owner’s manual or your mode’s repair guide.
  • Step Four: Drain out some of the engine coolants or completely drain the system as part of a complete radiator flush and fill.

You just don’t want to mess with leaking coolant while replacing the ECT sensor.

  • Step Five: Push on the tab and pull out the electrical connection to the ECT sensor. Make sure to keep the electrical connection clean and dry!
  • Step Six: Pull out the old ECT and double-check to make sure it matches the replacement ECT perfectly.

Depending on the model, you might need to use a wrench or a ratchet to remove the fasteners.

  • Step Seven: Check the repair guide for torque requirements for your ECT sensor. Most are around 20 lb-ft.
  • Step Eight: Insert the new ECT sensor and torque it to the required tightness.
  • Step Nine: Reattach the electrical connection and reinstall the plastic engine cover.
  • Step Ten: Complete your radiator flush and fill or add more coolant until it reaches the COLD line in the reservoir. Then, make sure to bleed any air out of the system.
  • Step Eleven: Reconnect the battery with the red (Positive) lead. Clear any codes in the computer and take the car for a test drive.

Frequently Asked Questions

Is It Safe to Drive with the Engine Temperature Warning Light On?

When the coolant temperature warning light turns on, you need to pull over as soon as possible, as the engine is dangerously close to overheating. If you’re in a bad spot but still close to home or your mechanic’s garage, you can wait 45 minutes to an hour for the engine to cool down. If it’s a cool day and you have a headwind, you might be able to drive another 15 to 20 minutes before the coolant temperature warning light comes on.

Can a Bad ECT Sensor Cause Engine Misfires?

When there’s a problem with the ECT sensor or the engine is running very hot, the ECU can struggle to maintain the right combustion conditions. This affects engine performance severely and can cause dangerous misfires, poor MPG, and black exhaust.

How Long After the Coolant Temperature Warning Light Comes on Before the Engine Overheats

Automakers usually set the threshold for the coolant temperature warning light coming on around 220 to 240 degrees. This is when overheating is imminent, and if you don’t pull over immediately, you will likely suffer some type of overheating damage to the engine. If you haven’t already. This is not the time to push the limits to get home a few minutes sooner.

Stay Cool, Stay Safe

When the coolant temperature warning light comes on, the engine is about to or already overheating, and you need to pull over as soon as safely possible. Pop the hood and have the patience to let the engine cool down. The coolant’s temperature will be over 220 degrees or more, which is too hot for you to troubleshoot anything under the hood.

Look under the car for signs of a coolant leak. Overheating can cause gaskets and seals to fail and hose clamps to loose. This can lead to low coolant.

Inspect the coolant for signs of sludge or debris when the engine has cooled down. If it’s dark and grainy, perform a radiator flush and fill. You might also have a clog in your radiator or your cooling lines.

If you have white, sweet-smelling smoke from the tailpipe, you likely have a blown head gasket or a cracked cylinder head. This is usually a major repair, but if the head gasket is only partially blown, you might be able to get by with a sealant additive for a while as you save up for the total replacement.

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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