The coolant temperature sensor uses a thermistor to measure the coolant temperature inside the engine block.

The coolant temperature is the basis for the calculation of the optimal fuel injection quantity and fuel injection timing.

A defective coolant temperature sensor may cause engine starting problems as well as higher fuel consumption and higher idling speed.

The coolant temperature sensor is in constant contact with the coolant and so there is a higher risk for it to corrode. Other failure reasons can be broken cables or mechanical damage caused by vibrations.