Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.
The engine coolant temperature (ECT) sensor test is simple and can help you fix your car faster. You can do it at home using a digital multimeter and a cooking thermometer. A bad engine coolant temperature sensor affects engine performance:
- It may cause a constant lean air/fuel mixture, causing the engine to stall or idle rough.
- Or it may cause a constant rich air/fuel mixture, causing an increase in emissions and fuel consumption.
- On modern vehicles, a bad ECT sensor will upset ignition timing.
- On some vehicle models, a bad ECT sensor may upset the transmission, cooling fan, and temperature gauge as well.
But before you blame the coolant temperature sensor for your engine problems, though, use this guide to test the sensor to confirm that you actually need to replace it. The test only takes a few minutes.
Tools and Items You'll Need
- Digital multimeter
- Kitchen thermometer
- Wrench (ratchet and socket set, if you need to remove components)
- Paper and pencil
Locate the Coolant Temperature Sensor
Open the hood of your vehicle to find the coolant sensor.
- Depending on your particular vehicle model, you may find the sensor mounted on the cylinder head or intake manifold. However, one of the most common locations is on or near the thermostat housing. You can find the thermostat housing by following the upper radiator hose, which connects to the thermostat housing on the engine side.
- Look for a large nut with an electrical connector on top, and two electrical wires coming from the connector — on most modern vehicles — or one wire — on older vehicle models.
- Some sensors are buried under the intake plenum, specially on large trucks, and you need to remove the intake just to gain access to it.
If you still have trouble locating the ECT sensor, consult your vehicle service manual. You can buy a service manual for your particular car make and model in most auto parts stores or online. Check the Amazon ad below.
Coolant Temperature Sensor Test
Now that you've located the ECT sensor on your vehicle, you're ready to troubleshoot it.
1. Unplug the sensor electrical connector.
2. Get the engine surface temperature using an infrared thermometer or suitable cooking thermometer. Take the engine temperature on a location near the coolant temperature sensor.
Before going on to the next step, let's divert here for a second:
Okay, at this point you may be wondering why you need to take the engine temperature to troubleshoot the sensor. The main reason is that you are trying to check two common, potential failures here, the ECT sensor and the thermostat.
Let's say that the thermostat on your vehicle got stuck in the open position. This will not allow the engine to reach operating temperature because the coolant is flowing continuously. If you were to test the coolant temperature sensor alone, you may think that it failed because its resistance value has remained at about 1500 or 2100 ohms, for example, when in fact the sensor is reporting the coolant actual temperature and it's working properly.
You could relay on the temperature gauge on your dashboard. However, on some vehicle models this gauge works through the ECT sensor as well. So, if the sensor doesn't work properly, your temperature gauge won't be of much help either.
By using the thermometer, it won't take you long to figure out that the thermostat isn't working. You'll notice the engine's temperature is not rising above 85 or 90 degrees, for example.
On the other hand, if the thermostat works fine, the engine temperature will reach about 200F (93C) and will drop afterwards as the thermostat opens. So you eliminate the thermostat as another possible failure.
Okay, now let's go on to the next step.
3. Take a note of the temperature reading.
4. Now, using your ohmmeter, measure the resistance value of the coolant temperature sensor by hooking up one of the meter's leads to one of the terminals on the sensor electrical connector, and the other lead to the other terminal on the sensor electrical connector.
On vehicles with old, single-wire sensors, hook up the meter leads to the terminal on the connector and the sensor's body (ground) to take your reading.
5. Check your vehicle service manual for the correct resistance value for your ECT sensor. However, not all service manuals have this information.
Most sensors of this type have a resistance value of 3000 ohms or more at about 55F (13C). You may want to try searching online for a resistance value table for your particular ECT sensor, if you know your sensor's brand.
However, whether you find the resistance values for your particular ECT sensor, continue with this tests anyway, the sensor's behavior and the temperature readings may give you a clue to its operating condition.
6. Make a note of the sensor's resistance.
Now you're going to take another pair of readings.
7. This time, start the engine and let it idle.
8. Set the transmission to Neutral and apply the parking brakes.
9. Wait for about one to two minutes and measure the engine temperature and the sensor's resistance as you did before.
10. Make a note of these new pair of values.
11. Without turning off the engine, wait for about one to two minutes and repeat this process again.
12. Take another pair of readings in about one to two minutes again, always noting the value readings.
13. Then turn off the engine.
On the next video, you'll see an alternative method to test a sensor using water. Of course you need to remove the sensor from the vehicle to use this method. However, the video will give you a visual reference about how to test your ECT sensor. Check it out.
Checking Your Readings
Now, compare your figures to the resistance and temperature ratings for your particular sensor listed in your vehicle service manual.
If you don't have the reference resistance values for your ECT sensor, make sure that your sensor resistance readings decreased proportionally as the engine temperature increased. This will indicate that your coolant sensor is responding to engine temperature.
Also, make sure the engine reached operating temperature at about 200F (93C) before dropping. If engine temperature didn't change much throughout your readings, the thermostat is stuck open and you need to replace it.
When should you replace the sensor?
- Whether you got an unusual reading or not, check that the wiring and electrical connector is free of corrosion. If one or more wires show sign of damage, repair them. Remove corrosion from the sensor's electrical terminal using electrical contact cleaner and repeat your test, if necessary.
- If the ECT sensor's resistance didn't change as the engine temperature rose, replace the sensor.
- If your sensor only registers infinite resistance, the sensor has a broken internal contact and you need to replace it.
- If the sensor only registers zero resistance, the internal contacts are shorted. Replace the sensor.
The coolant temperature sensor test is rather simple. It only takes a few minutes and you don't even have to remove the sensor from the engine for the troubleshooting procedure. Following the steps outlined in this guide will help you quickly determine whether you need to replace the sensor and will also check the thermostat operation at the same time by using a thermometer. Also, make sure to inspect the sensor electrical terminal for damage and corrosion, and check the circuit wires for possible damage. It won't help that your ECT sensor tests Okay when it has to operate with a damaged terminal or wire.
Test Your Knowledge of Failing ECT Sensors
For each question, choose the best answer. The answer key is below.
- Which is Not a Symptom of a Potential Bad ECT Sensor
- Continuous rich air-fuel mixture
- Continuous lean air-fuel mixture
- Rough idle
- Engine misfire
- Engine stalling
- Increased emissions
- Increased fuel consumption
- Engine misfire
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 replaced my ETC and now the gauge reads hot, what could this be?
Answer: If the gauge goes all the way into red as soon as you turn the key, there might be a bad electrical connection - check the wires (damage) and connections at the ETC, but in some cases you still need to look farther into the PCM or module, depending on your model. You may need to look the diagram for your model.
Question: I replaced the ECT twice. The temp gauge sticks all the way cold and fans are constant. It blows heat and doesn't overheat. Also, coolant is only warm to the touch when the engine has been running for an hour. Is my thermostat causing my temp sensor to keep failing?
Answer: There could be more than one issue here. You may want to check the temperature sensor circuit and the sensor. You may need your vehicle repair manual to trace the circuit in your particular model. Also, check the thermostat, if necessary. These two posts may help.
Question: Why is there pressure return to the coolant reservoir, and coolant to hot, but the radiator fan still won't turn on?
Answer: Test the temperature sensor. The fan can also fail, and wires in the circuit can break or become loose. Check the connectors as well.
Question: When idle, I’ve noticed the ECT climb up to 235 degrees F and continue. It starts going back down when I start moving again. How high can the engine get before it’s not normal?
Answer: It depends on the model, but a good operating temperature ranges at 195-225, including modern engines. If the temperature climbs when idling, you may want to check the coolant level, radiator fan and drive belt. Usually, these are the ones the cause trouble at idle.
Question: The reading from my coolant temperature sensor while plugged is different from the manual self-check I did. Does this mean that I have a bad PCM?
Answer: Compare your reading to specifications, and test both voltage and resistance, if possible. Do a cold and hot temp reading to double check.
Question: Why does temperature gauge go up to only 130 then drop back down continuously while the truck is started?
Answer: There could be a problem with the thermostat. Feel the radiator hoses while the engine is warming up and after a few minutes and see if the thermostat is opening.
Question: Why won't cooling fans engage when the A/C is turned on and the clutch is not engaged?
Answer: The system may be low on refrigerant. Without pressure, the system will not generate heat so that the cooling fans won't come on. There could be a problem in the circuit itself, perhaps on the high pressure. Also, have the A/C system serviced.
Question: Will my ECT cause cool air to blow?
Answer: The problem could be with the HVAC blend door. I would check the control unit or motor for the door.
Question: When the ECT is bad, should the trouble code definitely come or not?
Answer: Ideally, this should be the case. But it may differ in certain models depending on the enable criteria set for the computer. If the fault is within the electrical components or the circuit and the fault only appears for a second or less then a second, the computer may not register the fault, if not all conditions are met. This is also the case with some crankshaft position sensors on some models. In this case, you should look at the symptoms.
Question: What coolant temperature sensor do I use for a Daihatsu G200 car?
Answer: This video may give you an idea of the location:
Question: Will it hurt my motor driving it if it goes straight to red?
Answer: Yes, it can damage the block, cylinder head, head gasket and can result in very expensive repairs. Try to pull off the road if temperature gauge goes straight to red and shut off the engine. Let it cool and see if you can find out the cause of the problem or have your car towed to the garage.
Question: Can the coolant temperature sensor prevent a vehicle from starting?
Answer: It may cause a hard-to-start condition or cause the engine to stall after starting, since the computer won't know to add extra fuel for a cold start.
Dan Ferrell (author) on August 05, 2020:
I think these other two post may be of some help:
Victor on August 05, 2020:
Thanks for your response sir, but the reason for my question is that.. my low speed fan doesn't work, the high speed fan only comes up when my AC is turned on. But what bothers me is that the AC works intermittently.. it cools perfectly and goes off after few minutes. I noticed that it goes off whenever there is a change in speed or during high temperature. I ve taken the AC for a check, the technician said my AC is working perfectly.. he advised I solve the low speed fan issue first and get back to him.
Hence my question of ECT affecting AC system of a car.
Dan Ferrell (author) on August 04, 2020:
The ECT sensor shouldn't affect the AC system. This vvideo can give you an idea for your three-wire sensor test.
Victor on August 03, 2020:
1. Can ECT sensor affect the air conditioning system of a car?
2. How do I test a 3 wire ECT?
Dan Ferrell (author) on November 21, 2019:
There may not be a problem for the computer, but having the fan running all the time is inefficient. At 30 mph and above airflow is better through the radiator and you don't need the fan running. But the fan's service life may shorten drastically. It's not designed to run all the time. If there's an overheating problem, it's better to have the system checked and fix it.
Emmanuel on November 21, 2019:
I had a car electrician connect the fan of my toyota venza to a relay in other to keep the fan always running. Would it cause any trouble to the computer ?
Dan Ferrell (author) on May 07, 2019:
You are looking for either the coolant temperature sensor or the sending unit. Make sure you know what you need, and have your model specification with you. If in doubt, use also your VIN number. You can see the number through the windshield on the driver's side. Or the certification label on the left front door pillar. Hope this helps.
Deanna on May 07, 2019:
I have a 1998 Ford Explorer 4 wheel drive and im looking to replace the coolant temp. Sensor and was wondering if its not that is it the PCM SENSOR
Dan Ferrell (author) on April 21, 2019:
Temperature rises for a few minutes after shut off, it is normal since cooling system isn't working. Engine operating temperature range is within 195-220. I think you're engine is doing OK.
Andrew peyre on April 19, 2019:
I have a 2004 Chevy blazer my engine doesnt over heat while im driving or idling but once its warmed up when i shut it off for a few min then start it back up the temp spikes to like 230 degrees once it starts idling it cools back down to between 190 and 210 could it be the sensor doing that
Dan Ferrell (author) on September 29, 2018:
Sorry I can't be of any more help. But you may try the diagram for your model (probably a Haynes book). Getting to the gauge is tricky but the manual may give you some ideas.
ron from pocatello, id on September 29, 2018:
Thanks for your reply, but the sensor is fine, my problem is the wire is broken between the temp sensor and the gage, i have two other wires broken because i moved the wire bundle around fixing something else, i have the other wires figured but the temp sensor has several wires going to the gage and if anyone has delt with this type problem on the temp circuit i was hoping for some ideas dealing with that circuit. Wiring is a kind of nightmare on cars nowdays and trading info can be quite bennificial. thanks again ron
Dan Ferrell (author) on September 27, 2018:
This post my help you
Brian on September 26, 2018:
I have a 97 Ford Courier 2.6lt petrol. Using about 17 lt per 100km. Injectors reconditioned. New Mass Air Flow Metre. Idling rougher after MAFS replaced. Coolant sensor replaced. Any ideas for better fuel economy
Dan Ferrell (author) on August 15, 2018:
If the problem is at the gauge end, the ecm might still be getting the signal from the sensor. The gauge connects to the sending unit. You may need to have the electrical diagram found in the repair manual for your vehicle to id the wires and connections to make sure they're still connected properly. If you don't have the manual, your local library may have a copy you can consult.
ron from pocatello, id on August 15, 2018:
i was working under the dash on 92 volvo 960 and i lost the coolant temp gage and cruise control, do you have any suggestions in tracking the problem. I also question if the ecm is still getting a signal even though the gage shows nothing. thanks ron
Dan Ferrell (author) on April 07, 2018:
The operating temperature is set at 195 degrees in theory, but it'll fluctuate depending on operating conditions and load. Nothing to worry about if the system is operating properly.
Dan L. on April 07, 2018:
Nice post! Got a question though that I haven’t gotten a straight answer to yet. On my 08 Grand Caravan 3.3 engine, what would be the proper operating temperature of the coolant?
Dan Ferrell (author) on March 21, 2018:
Check the specs in your vehicle repair manual. If you don't have a copy your local library may have one in the reference section.
M. C. Zonkwa on February 12, 2018:
Dan Ferrell (author) on February 07, 2018:
Make sure the sending unit is working properly, test it when cold and hot. Check the connections are clean and tight from the sending unit to the gauge.
If necessary, test the gauge as well.
Have you checked that the engine is not actually getting hot?:
* radiator filled with grime,
* cooling fan working properly
* good radiator cap
* water pump
Marty Conley on February 06, 2018:
I have been having a problem with the temp gauge in my 99 chevy pu. with a 7.4 engine. It reads normal for about 15 to 20 min. and then it goes all the way over to hot. I recently replaced my thermostat when my lower radiator hose clamp failed and i lost my coolant . I have tested my thermostat, coolant temp sensor, and temp sending unit. All show to be good.
Dan Ferrell (author) on November 28, 2017:
If the thermostat is stuck open the engine will run below operating temperature.
Austin on November 27, 2017:
I noticed that I needed to change my temp. sensor when it was 35° out side I started the car and my temp hand went to hot! Just as I started the car. I replaced the sensor but would the sensor make the car run cool? The temp.hand shows that it is cool! I have hot air. But I mean the car run's very cool it also does not make it run rough