What is an engine coolant temperature sensor?
An engine coolant temperature sensor can be
located near the engine thermostat or anywhere in the engine cooling system such as the cooling jacket, cylinder head or the radiator. Its job is to report the temperature of the engine. The engine coolant temperature sensor reports its findings directly to the powertrain control module or engine control module. The PCM/ECM uses the engine coolant temperature reading to calculate how much fuel to add to the incoming air.
How does an engine coolant temperature sensor work?
Most engine coolant temperature sensors are either positive temperature coefficient or negative temperature coefficient thermistor. The PCM/ECM supplies a voltage to the sensor and the sensor alters the incoming voltage by applying a varying amount of resistance based on the air temperature.
A negative temperature coefficient thermistor decreases resistance as the temperature increases, while a positive temperature coefficient thermistor increases resistance as the temperature increases.
If the PCM/ECM supplies a 5-volt input signal, it should see a return voltage as shown below
Positive Temperature Coefficient
Engine coolant temperature sensor
Temperature ° F Voltage
-40° F 4.90 V
+33° F 4.75 V
+68° F 4.00 V
+100° F 3.00 V
+143° F 2.00 V
+176° F 1.30 V
+248° F 0.60 V
+305° F 0.0 V
What goes wrong with the engine coolant temperature sensor?
Like any other sensor, the sensing element can fail, the terminals in the electrical connector can corrode and alter the readings, or the wiring harness can develop a short or open.
How to test an engine coolant temperature sensor?
You can test an engine coolant temperature sensor using a digital Ohm Meter set on the DC volts scale. Turn the IGN switch to the ON position and backprobe the return wire to see the voltage being reported to the PCM/ECM. You can also test the sensor’s resistance, but that’s not as accurate as reading the actual return voltage.
How to replace an engine coolant temperature sensor?
Engine coolant temperature sensor (IAT) sensors can be screwed into the intake manifold or simply pushed into a rubber grommet. Remove the old sensor and install the new sensor in its place.
Symptoms of a faulty engine coolant temperature sensor
Engine will crank but fails to fire up on a cold start first thing in the morning. The incorrect engine coolant temperature reading causes the PCM/ECM to provide too lean a mixture for the current engine temperature.
Engine cranks but will only start if you depress the gas pedal part way. Depressing the gas pedal overrides the factory programming and forces the PCM/ECM to add gas to the mixture. If the engine starts with the pedal depressed, suspect a faulty engine coolant temperature sensor or a fault in the sensor wiring.
Poor gas mileage
©, 2018 Rick MuscoplatPosted on by Rick Muscoplat