What is the engine temperature sensor? what does it do? What does it look like and where is it fitted?? All is explained in part 1 of this series on engine coolant temperature sensors.

In a car with an internal combustion engine, the cooling system maintains the optimal temperature of the engine and prevents it from overheating.

An engine coolant temperature sensor or ECT measures the temperature of the liquid coolant. A typical engine cooling temperature sensor is a Negative Temperature Coefficient (NTC) thermistor, which means its electrical resistance decreases when the temperature increases. The tip of the ECT sensor protrudes into one of the cooling system passages and is immersed in coolant.

Many cars have more than one coolant temperature sensor. In most cars, the primary ECT sensor (ECT sensor 1) is installed near the thermostat in the cylinder head or block or on the thermostat housing.

A second coolant temperature sensor could be installed in another part of the engine, or in the radiator.

Some cars use a cylinder head temperature or CHT sensor instead of, or in addition to the ECT sensor. The CHT sensor works the same way, but it measures the temperature of the cylinder head metal and is not immersed in coolant. This allows the CHT sensor to measure the temperature of the engine correctly even with loss of coolant. In some cases, this may help prevent overheating.

An ECT sensor is connected to the main computer (powertrain control module PCM or Engine control module ECM or Engine control Unit ECU). The PCM supplies a reference voltage (typically 5 Volt) and constantly monitors the ECT sensor signal. Based on this signal, the PCM adjusts the engine performance and operates the electric radiator fans when the temperature reaches a pre-determined level.

If the signal from the sensor is missing, or it is outside of the expected range, the PCM turns on the Check Engine light and stores the related trouble code in its memory.

One of the common problems is when the sensor has poor connection inside or in the connector or in the wiring harness. This causes interruptions in the signal to the PCM and the PCM sets the fault. In some cars, symptoms of this problem show up as erratic readings of the temperature gauge. The engine may run in fail-safe mode: The air conditioner may stop working and the radiator fans may run constantly.
For example, as per the service bulletin 05-002-12 for the 2011 Dodge Journey with the 2.4L engine, to repair the trouble codes P0117 (Engine Coolant Temperature Sensor Circuit Low) or P0118 (Engine Coolant Temperature Sensor Circuit High), the brake booster vacuum hose that was routed too close to the ECT sensor connector must be replaced.
The GM service bulletin PI0631F for the 2012-2013 Chevrolet Impala mentions the issue with the wiring harness rubbing on the front right side of the transmission. If any of the wires are damaged or shorted, it could cause many different trouble codes, including the ECT sensor codes P0117 and P0118. The harness must be repaired to correct the problem.

Corrosion at sensor terminals or connector can also cause sensor-related faults. For example, the Ford service bulletin TSB 11-10-5 describes the problem in the 2010-2012 Fusion, Escape, Transit Connect, as well as Mercury and Lincoln branded vehicles with the 2.5L engine: water contamination in the sensor connector can cause codes P1285, P1299 and/or P0128. Depending on the severity of the corrosion, the connector and the CHT sensor must be cleaned or replaced.

The bulletin SI M17 06 12 for some R55, R56, R57 and R58 MINI Cooper/CooperS vehicles describes a similar problem with the corrosion inside the engine coolant temperature (ECT) sensor that can cause inaccurate engine temperature reading. The repair involves installing a new sensor and retrofitting a few related parts.

Many Check Engine light codes related to the ECT sensor could also be caused by other reasons, such as a bad thermostat or issues with the cooling system, including even a leaking head gasket. The problem must be properly diagnosed. Of course, considering that the ECT sensor is not an expensive part, it's often recommended to be replaced if suspected to be malfunctioning.

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