How do you know if your coolant temperature sensor is broken?

Answer 1

GTIs with K-Jet mechanical injection (8-valve engines up to 1987 and 16-valve engines up to 1992) only have a single Coolant Temperature Sensor which is connected to the dashboard temperature gauge. A failure of the CTS will result in the gauge reading maximum or minimum regardless of true engine temperature.

GTIs with electronic fuelling (8-valve engines after 1987 and 16-valve engines after 1992) have two CTSs, a black sensor which is connected to the dashboard temperature gauge, and a blue sensor which is connected to the engine management ECU. A failure of the black CTS will result in the gauge reading maximum or minimum regardless of true engine temperature. A failure of the blue CTS will result in incorrect values being sent to the ECU and the fuel delivery will be affected, generally resulting in difficult starting when the engine is cold.

The black CTS isn't critical to the running of the engine and only provides information to the driver. The blue CTS iscritical and should be replaced if there are any doubts about its condition, particularly as they are an inexpensive part and easy to fit, and considerably cheaper than the first hour of labour at a garage.