Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.
A bad engine coolant temperature sensor (CTS or ECT) can seriously disrupt the fuel system and engine operation of your car.
That's because the ECT sensor tells the Electronic Control Module (ECM—your car computer) at what temperature the engine is operating, so the computer can make some important decisions. For example:
- how much fuel the engine needs
- when to go into closed loop operation (to monitor and adjust engine operation)
- whether to advance or retard the spark (for best ignition system operation)
- and make other adjustments for optimal engine performance
As you can see, a bad CTS can really mess up engine performance:
- A permanently closed (electrically shorted) coolant sensor will cause an increase in fuel consumption and potential damage to the catalytic converter.
- A permanently open (broken circuit) coolant sensor will starve the engine of fuel.
Luckily, this is a simple repair on most vehicle models. You can replace a bad CTS in your car following a few simple guidelines and precautions. After confirming that your coolant temperature sensor has failed, follow the steps in this guide to restore engine performance and save in car repair costs.
OK. So now the question is, can you replace the CTS yourself without much experience in car repair?
Well, that will depend on your particular vehicle model. Replacing the sensor itself is easy, but getting to the sensor is another matter. The good news is that, on most cars, the sensor attaches to the cylinder head or block where you can have easy access to it. However, on some models, you may need to remove large components like the intake manifold to reach it. This usually happens with V-engine models. Even so, you may still be able to replace it at home. So keep on reading.
Potential Symptoms of a Failed ECT
Two Important Facts About ECT Replacement
Potential Symptoms of a Failed ECT
Although the next indicators may point to a bad ECT sensor, problems in a different system may also show the same symptoms. That's why it's important to troubleshoot your coolant temperature sensor before installing a new replacement.
- Engine hard to start
- Driveability problems
- Diagnostic trouble codes: P0117, P0118, P0128 (these will show up on some models)
- Poor fuel economy
- Cooling fan fails to run (on some models)
Coolant Temperature Sensor Replacement
OK. So now you're ready to replace the ECT sensor.
1. Park your car in a safe place where you can work comfortably and let the engine cool.
2. Place a clean drain pan under the radiator, open the radiator valve and drain about two to three quarts of coolant (or just enough to drop the level below the sensor). Then close the drain valve. This will minimize coolant waste when you remove the CT sensor.
3. Locate the CT sensor.
- On most inline engine models (four or six cylinders), look for the sensor around the upper section of the engine block or around the cylinder head.
- On most V8 (and some V6) engines, you may find it also around the upper part of the engine or under the intake manifold.
- Look for a one-inch diameter (approximately) nut with a two-wire electrical connector on top.
NOTE: A couple of suggestions here. If you can't locate the sensor, consult the vehicle repair manual for your particular car make and model. Also, even if your manual says that you have to remove a large component to access the sensor (such as the intake manifold) look for other, potential components you may remove instead to reach the sensor. You may find an alternative way to reach the sensor, if you have the correct tools. Your repair manual will have instructions on how to remove whatever part you need to remove.
4. Once you've reached the sensor, unplug the sensor electrical connector. Be careful when disconnecting the sensor. You may need to press a tab or pry loose a retainer wire before you can unplug the connector. Some sensors use a mounting clip you can pry off using a thin screwdriver or scratch awl.
5. Use a six point wrench, deep socket or line wrench for a firm grip.
6. Compare the new CT sensor to the old one; both should have the same configuration.
7. Depending on your particular application, install a new sealing washer, or apply a light coat of an approved sealer, or wrap Teflon tape around the threads of the new sensor (give it a couple of turns). This will keep coolant from leaking. Consult your car repair manual, if necessary.
8. Hand start and finger tight the sensor in the engine. Then tighten the sensor with a six-point wrench, deep socket or line wrench. Torque the sensor to the specification listed in your repair or service manual. Apply the correct torque: Overtightening the new unit may ruin the threads or bottom out the sensor against the engine and break it.
9. Plug in the coolant sensor electrical connector.
10. Reinstall any components you may have had to remove to access the CT sensor.
11. Replace the coolant you previously drained, or refill the radiator with fresh coolant.
12. Start the engine and let it idle. Wait for the engine to reach operating temperature.
13. Check for coolant leaks around the sensor threads.
The following video will give a visual idea on replacing the coolant temperature sensor.
Two Important Facts About ECT Replacement
- Keep in mind that not all coolant temperature sensors are made equal. ECT sensors differ somehow in operation and rating. Buy the correct sensor for your particular vehicle make and model. Otherwise, your car computer will work with incorrect data for your engine and driveability problems will persist.
- When replacing the sensor, closely check the weather seal at the sensor electrical connector. This weather seal is easy to misalign when you plug back in the connector, which will cause the new sensor connector to rust and fail sooner than expected.
If you have decided to replace the CT sensor and your car's service schedule is coming up, you might want to replace the coolant in the system as well, since you have to remove some of the coolant. Also, for a more complete repair job project (and if you haven't changed it in the last four or five years), you may want to replace the thermostat as well. A thermostat will eventually fail, and you'll find yourself removing the coolant again soon.
Check the vehicle repair manual for your particular vehicle make and model. It'll have the correct coolant to use for your vehicle, identify the correct components and location and offer you the steps to follow for the maintenance job.
Replacing the CTS yourself can save you about $150 in repairs or more, depending on your particular vehicle model. And the best part is that, for most models, the sensor is readily accessible, so the average car owner can perform this repair at home using common tools. All you need is one or two hours of work in your garage on a Saturday morning. And if you want to keep your cooling system—and engine—working at its best, don't forget to check the system periodically and service it at your manufacturer-recommended intervals.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Question: I installed a new temp sensor in my ‘99 Mercury Grand Marquis, but the gauge in the instrument cluster always reads cold. What can I do to fix that?
Answer: Make sure the thermostat is working. Conduct a simple test, after driving for 20 minutes or more and the cooling fan has come on, open the hood and feel the upper radiator hose. Be careful here. It should be hot. If the hose doesn’t feel hot, possibly the thermostat is stuck open and not letting the engine reach operating temperature, in which case your temperature gauge is working fine. Otherwise, the gauge isn’t working or there’s a problem with the circuit.
Question: I have Nissan Rogue. I just replace the thermostat and coolant sensor, but every time I drove 20 minutes with ac on and about 70 mph the coolant sensor warning always goes on saying the car is overheating, but the engine coolant gauge is normal. What could be the problem?
Answer: There could be a problem with the AC system, possibly a bad switch.
Question: My 1998 Buick Century was running, but then a light comes on the dash that says "Hot Check All." It says it's heat sensor but the car will not restart. What should I do?
Answer: On some models, the computer will prevent the engine from starting when coolant issues are present. Test the coolant sensor and replace it, if necessary. This post might help:
© 2016 Dan Ferrell
Dan Ferrell (author) on May 07, 2018:
Scan the computer for codes again and see if there're any pointing again to the TPS or something else.
Other possible issues that can lead to a high idle speed:
* Bad idle air control (IAC) valve/solenoid or buildup preventing the valve from proper operation - this valve is accesible and easy to test. You can even remove it for inspection. If you have the repair manual for your vehicle model (or check it at the local library) you may be able to do it yourself.
* Take a look at the throttle body, manually work the throttle valve and look for buildup around the bore and underneath the valve - if there's buildup, clean it with carburator cleaner. Buildup can prevent the valve from operating propoerly and cause high idle as well.
* Check that the PCV valve is not clogged or one of the connections or hoses are loose or damaged. The valve connects to the valve cover and is accessible as well.
* Make sure the air intake ductwork is properly sealed - this containes the air filter - if there is something loose or note seal it can cause unmetered air to enter and upset engine idle.
* Check for vacuum leaks - inspect evey vacuum hose for damage or loose connections.
* A bad mass air flow sensor (MAF) would triger the check engine light - it there's a trouble code for it, clean the MAF sensor first, this is a common issue with this sensor, or it could be bad. The sensor is located near the air cleaner assembly and is accessible as well.
These are the most common issues the may cause the engine to idle high.
Michael Moore on May 07, 2018:
I took my 03 Malibu had them pit the computer on it and only thing that came up was bad tps sensor had a new one put on and it stopped idling high for little while then started doing same thing guy said only other thing it could be is throttle body really can't afford 400 bucks for it tho so don't know what I'm going to do
Dan Ferrell (author) on February 03, 2018:
There could be many reasons for an engine surging,
the most common ones being a vacuum leak, a faulty IAC solenoid (idle air control), a bad TPS (throttle position sensor).
Scan for possible pending trouble codes, if you haven't done so.
Jim on February 02, 2018:
hey dan i have a 89 ranger with 2. 9. The problem is it idles at 12 to 15 00 rpms. When driving down road at f55 to 70 the rpms goes well over 3000 to 3500 rpm and then drops and then back up and keeps fluctuating. Idles real high even after warmed up. Any ideas.