Coolant Temperature Sensor

It is one of the most important engine sensors. When the engine is cold, the control unit enriches the mixture and raises idle speed; as it warms up, it leans out. The sensor also decides when the radiator fan switches on and, on many cars, feeds the dashboard gauge.

Most sensors are thermistors whose resistance falls as temperature rises. The control unit sends a reference voltage and reads the voltage drop to calculate temperature. Many engines have more than one temperature sensor: one for the control unit, sometimes a separate one for the dashboard gauge, and on some engines another at the radiator outlet.

The sensor tip sits in the coolant flow, often near the thermostat housing. The control unit converts its signal into a temperature and checks it for plausibility against other data, such as how quickly the engine should warm up. The sensor is usually a thermistor whose resistance falls as temperature rises. The control unit reads it as a voltage and turns it into a temperature value.

A faulty sensor can cause hard starting, high idle, poor economy, black smoke or a fan that never stops, and sets codes such as P0117 or P0118. A slow warm-up code (P0128) more often means a thermostat stuck open. A sensor that wrongly reads cold can cause high fuel consumption, black smoke on diesels and hard hot starts; one that reads hot can switch on the fan constantly. Live data on a scan tool shows whether the reading is plausible.