How to Troubleshoot a Car Engine Temperature Gauge

To troubleshoot your car's temperature gauge, you need to know how it works.

The temperature gauge reading starts out as a reference voltage that is sent to the coolant temperature sensor. This sensor is nothing more than a thermistor -- a variable resistor that changes resistance with temperature changes. As the coolant temperature rises or falls, the resistance of the sensor changes as well. On most vehicles, the resistance of the sensor decreases as the temperature increases. At low engine temperatures, almost all of the reference voltage is dropped as the current passes through the sensor, and when the engine is warmer, very little voltage is dropped. Depending on how your vehicle is designed, that modified voltage travels directly to the temperature gauge -- more common on cars built before 1996 -- or back to the car’s computer, which then controls the temperature gauge in the instrument cluster.

To diagnose an inoperative temperature gauge, you’ll need to test the coolant temperature sensor and the wiring for that circuit. You should have a service or repair manual written for your specific vehicle prior to attempting these tests. You will need to drain and fill the cooling system as well as test specifically colored wires. Removal of other components may be necessary to access the coolant temperature sensor, the sensors wiring or the wiring for the temperature gauge. The following procedure will work for most vehicles, but check the repair manual for a description of the sensor’s operation before you begin.

Find the coolant temperature sensor.

The location of the sensor varies depending on what year, make and model of vehicle you are working on. It is commonly found threaded into the engine block, thermostat housing, cylinder head or upper intake manifold.

Check for reference voltage to the sensor.

Disconnect the wiring harness from the sensor. It may have two or three wires. Review your repair manual to determine which wire supplies the reference voltage. Set your multimeter to read DC volts, on the 20-volt scale. Connect the negative lead of the multimeter to a good ground -- ideally the negative battery terminal or one of the body-to-engine grounds. Touch the positive lead from your meter to the reference supply wire. You should see 5 or 12 volts with the ignition key in the “On” position, depending on how your vehicle is designed. Proceed to the next step, if you receive a voltage reading. If you do not get a reading, check any fuse labeled “Engine,” “ECM” or “PCM.” If all fuses are good, trace that wire back to the computer and repair any damage to the wire. If there is no damage to the wire or any blown fuses, the computer is likely at fault.

Test the coolant temperature sensor.

Follow the steps in your vehicle’s repair manual to release the pressure in the cooling system, and then drain the coolant.

Remove the temperature sensor with a deepwell socket and a ratchet. Attach alligator clips to the leads of your multimeter. Fill a pot or pan with room temperature water and place it on one of your stove’s burners. Attach the multimeter leads to the two pins in the sensors connector, then submerge the tip of the sensor into the water. Set your multimeter to the Ohms setting on the 1K scale and turn on the burner. Observe the resistance of the sensor as the water temperature increases. The sensor resistance should start out high and drop as the water temperature increases. Check your repair manual for exact specifications and compare them to your readings. If the sensor falls out of specification, replace it. If the sensor tests good, clean the threads with a wire brush, apply new thread tape to the threads and install the sensor. Tighten the sensor to specifications given in your vehicle's repair manual. Fill and bleed the cooling system according to your vehicle's repair manual.

Check the sensor return wire

Review your vehicle's repair manual to determine if the sensors return wire runs directly to the instrument cluster or back to the computer. If it goes back to the cluster, follow the directions in the manual to remove the instrument cluster. If it goes to the computer, follow directions to access the computer. Set your multimeter to the 20-volt, DC-current scale and probe the return wire at the instrument cluster harness or the computer harness. You should see a voltage lower than the initial reference voltage you found in Step No. 2. If you do not get a reading, check for damage to that return wire and repair it as necessary. If your sensor reports directly to the instrument cluster, and you do not receive voltage, replace the instrument cluster. If your gauge is computer controlled, check for return voltage on that wire at the computer harness. If there is no output voltage from the computer, replace the computer. If there is voltage being sent from the computer to the instrument cluster, check for that voltage at the cluster. If you get voltage at the clusters wiring harness, replace the instrument cluster. If you do not see voltage at the cluster, but you got a voltage reading at the computer, inspect that wire and repair as necessary.