Checking the coolant temperature sensor – symptoms, cause of failure and troubleshooting

Coolant temperature sensors are used by the fuel management system to detect the engine's operating temperature. Depending on the sensor information, the control unit adapts the injection time and firing angle to the operating conditions. The sensor is a temperature sensor with a negative temperature coefficient. This means that the internal resistance reduces as the temperature increases.

The resistance of the temperature sensor changes depending on coolant temperature. As the temperature increases, the resistance is reduced, which reduces the voltage at the sensor. The control unit evaluates these voltage values, since they are directly related to the coolant temperature (low temperatures result in high voltage values at the sensor, and high temperatures result in low voltage values).

A faulty coolant temperature sensor can manifest itself in different ways through fault detection by the control unit and the resulting emergency program strategy.

Frequent fault symptoms are:

• Higher idle speed
• Increased fuel consumption
• Poor starting behaviour

In addition, problems may also occur with the exhaust gas test cycle through increased CO values or intermission of the lambda control. The following entries can be stored in the fault memory of the control unit:

• Short circuit to frame in the wiring or short circuit in the sensor
• Positive fault or break in wiring
• Implausible changes in signal (signal jump)
• Engine does not reach the minimum coolant temperature

The last fault code can also occur if the coolant thermostat is faulty.