- Coolant temperature sensors are resistors and can’t be reset. The engine control unit is the part that actually runs software and just takes information from the sensor.
- If you think you have a problem with your sensor, test its resistance by attaching it to a multimeter and submerging the sensor in hot water. If the reading goes down, it’s working.
- If your sensor isn’t working properly anymore, carefully remove the old one from the thermostat housing in your engine, unplug the top, and replace it with a new one.
Can you reset a coolant temp sensor?
No, the coolant temperature sensor cannot be reset. When you reset a computer, you stop any software that’s running and erase the stored data. The engine coolant temperature sensor, or ECT is just a resistor affected by temperature—nothing’s being stored on it and it has no software. The ECT works by receiving a signal from a computer called the engine control unit, or ECU. The ECU sends 5 volts into one terminal of the ECT, which is a negative temperature coefficient resistor, then reads the voltage coming out of the sensor. All this means is that when the sensor gets hot it sends out less voltage, and when it’s cool it sends out more.
How to Replace an ECT Sensor
Wait for the car to cool down completely. The coolant temperature sensor is plugged into the coolant stream of the engine, so it’s actually in the pressurized liquid. When you remove the sensor, there could be some coolant spitting at you, so wait at least two hours after driving to take it out. To keep the coolant from leaking out too much, let off some of the pressure by removing the coolant cap, then make a new vacuum seal by closing it again. The coolant reservoir is a large plastic container mounted in the engine bay.
Locate the sensor. In different cars, the ECT sensor is in different places. Look at your car’s manual to find out where yours is. Usually, it’s found near the thermostat in the cylinder head at the top of the engine. The ECT is a small plug, often with a green or black head. One side of it is in the coolant stream of the engine, and the other side has wires running out of it to the ECU. The ECT can sometimes be hidden under wires and tubes running over your engine, but should be accessible by pushing them to the side. Check the top and the sides of your engine for the sensor.
Pull off the spring clip with a flathead screwdriver. The spring clip is at the base of the ECT and holds it in the coolant stream. Put a large piece of tinfoil under the ECT to catch the clip if it falls. Most new ECT sensors come with a spare spring clip and O ring, but you can get them separately if yours doesn't.
Remove the ECT and O ring. Keep your face away from the coolant as you do this in case it comes shooting out. Pull the sensor out, then unplug the other end from the wires. After you remove the sensor, take off the O ring left on the connection to the engine. If you don’t see the O ring, it means it’s stuck in the ECT sensor. Use a pick (or bend the end of a nail at 90˚ and use that) to dig it out of the inside. Coolant is very sweet and very poisonous, so clean up any spills promptly to ensure no animals come in contact with it.
Reverse the previous steps to put in the new sensor. Plug the top of a new engine temperature sensor back in, then put an O ring around the base. Push the base back into its housing on the engine, then put the new spring clip on around the sensor. Turn on your car and let it run for a minute, checking if the problems have been resolved. If everything’s running well, reset your check engine light by disconnecting and reconnecting the battery.
Signs Your Coolant Temperature Sensor is Bad
The engine has difficulty starting cold. When your engine is cold, it needs extra gasoline to stay running until it warms up. When an ECT sensor is bad, the ECU can’t account for the temperature and won’t inject enough gas to start up normally. The injection problems can also cause your car to idle irregularly or have difficulty accelerating. ECT malfunctions aren’t the only reason you could be having this problem: it could be a faulty fuel injector, a broken idle air control valve, or a leak in your exhaust gas recirculation system.
The engine temperature gauge gives strange readings. A fully functional engine heats up to the middle of the temperature range on the gauge and then stays there. When the ECT is malfunctioning, the gauge can start giving you weird readings, like being on hot when the engine starts or staying on cool as you’re driving. A good sign that your ECT is acting up is when the needle of the gauge stays in one place the entire time you’re driving. If your driving isn’t affected (i.e. you don’t hear your engine struggling), you may just have a problem with the temperature gauge itself, and not with a part in the engine.
You’re getting bad gas mileage. If your mileage just plummeted, it may mean your ECT isn’t working properly. When the engine is cool, the ECU relies on the ECT sensor to know how much gas to give, then it switches over to more accurate oxygen sensors when it gets hot. If your ECT sensor is broken, the switch won’t happen. Lowered gas mileage can also happen when the injectors are bad, the oxygen sensors malfunction, or even when your tire pressure is low.
Your exhaust is black or smells like fuel. When the ECT malfunctions, the ECU can inject too much fuel into the combustion chamber, which makes the air smell gassy and produces black smoke. Gassy-smelling exhaust can also be caused by leaks in your exhaust system.
The check engine light is on. If your check engine light goes off and you already suspect your ECT sensor is having issues, go to an auto shop to get the codes read. Most places will do it for free. If you have one of the following codes, it indicates a problem with your coolant temperature sensor: 17704, error in the mapped cooling system 01039 ECT sensor RPM at idle is irregular p1296 35-00 ECT sensor or thermostat malfunction p0118/16502 ECT sensor circuit high input
How to Test Your Sensor
Check for visible damage. The ECT sensor is in different places in different cars—some even have multiple sensors. Check your manual to find out where yours is, and if there are two, look for the one on the thermostat housing. When you find it, perform a visual inspection to look for loose connections and broken wires. There are a few types of ECT sensors. Most will look like a small plug coming out of the coolant stream or the thermostat housing, and all will have wires coming off the top leading to the ECU.
Check that the fan turns on at 97 °C (207 °F). Open your hood and take off the radiator cap. Use an infrared thermometer to measure the coolant temperature, then start the engine. The coolant will start warming up, and at 97 °C (207 °F), the fan should start running. If the power doesn’t turn on, something’s wrong with the ECU or the sensor.
Check the ECT ground. To function properly, the ECT sensor needs to be grounded to the electric control unit, or ECU. Connect one probe of a multimeter to the positive terminal of the car battery. Stick a paperclip in the ECT ground terminal and connect the other probe to it. Check your manual to see which wire leads to the ground terminal. Clip the other end of the multimeter to the paperclip, and change the settings to “Voltmeter.” Turn on the car, but not the engine. If the ECT is properly grounded, the display should read around 12 volts when you check the multimeter.
Check the reference voltage. The ECT receives a constant 5-volt charge which it changes (through its resistance) and feeds back to the ECU. To check if the sensor is receiving the proper charge: Disconnect the wires from the top of the ECT sensor. Put one probe of a multimeter in each terminal of the connecting wires from the ECU, then turn on the electricity. If the meter reads 5 volts, you may have a faulty sensor. If it reads something else, the problem’s in the ECU.
Test the sensor’s resistance. Fully disconnect the sensor from the car, then connect the wires coming from each terminal to either probe of a multimeter. Put the sensor in a cup of boiling hot water, then move it to a cup with ice water. Watch the voltage as you move the sensor between the cups. It should go down in the hot water, then go up in the cold. According to Ohm’s law, voltage increases when resistance increases, so the higher the temperature, the lower the resistance and the lower the voltage.
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