Understanding the Role of an Engine Coolant Temperature Sensor

Learn what an engine coolant temperature sensor does, how it works and fails

An engine coolant temperature (ECT) sensor is a critical component in modern

Typical Engine Coolant Temperature Sensor

engines. It measures the temperature of the engine’s coolant, which is essential for the engine control module (ECM) to manage fuel injection, ignition timing, and cooling system operation. The ECT sensor helps ensure the engine operates efficiently and within safe temperature ranges.

How an engine coolant temperature sensor works

Most engine coolant temperature sensors are either a positive temperature coefficient or a 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
• A positive temperature coefficient thermistor increases resistance as the temperature increases.

Let’s use a real-life example. The PCM/ECM supplies a 5-volt reference voltage to the ECT. If the vehicle uses a Positive Temperature Coefficient ECT, the ECM expects to see the following voltages at these coolant temperatures:

-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

How the ECM uses data from the ECT

• Fuel delivery— The ECT sensor helps the ECM adjust the air-fuel mixture based on the engine’s operating temperature. For example, a cold engine requires a richer fuel mixture to ensure smooth starting and operation, while a warm engine needs a leaner mixture for efficiency.

• Ignition Timing— The sensor data allows the ECM to optimize ignition timing. In a cold engine, the ECU may advance the timing to ensure better combustion, while in a warm engine, it may retard the timing to prevent knocking and improve efficiency.

• Cooling System Control— The ECT sensor data is used to control the operation of the radiator fan and, in some cases, the thermostat. For instance, the ECM can activate the radiator fan when the coolant temperature exceeds a certain threshold to prevent overheating.

• Emissions Control— The ECT sensor helps maintain the correct engine temperature for optimal combustion, which reduces harmful emissions. It also assists in managing systems like the exhaust gas recirculation (EGR) system and catalytic converter operation.

• Engine Warm-Up— The ECT sensor data informs the ECU when the engine has reached its optimal operating temperature. This information can be used to adjust idle speed and deactivate cold-start enrichment systems.

Where the ECT is located in the engine

Carmakers typically locate the ECT near the engine thermostat or anywhere on the top of the cylinder head or the top of the radiator.

The engine coolant temperature sensor is usually located near the thermostat housing

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.

Symptoms of a faulty ECT

The 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 a mixture that is too lean for the current engine temperature.

The engine cranks but will only start if you depress the gas pedal partway. 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 Muscoplat