Thermostat - Fault Table

Symptom/
Cause/effects

A. The temperature gauge rises slowly, operating temperature is reached late, and heater output is insufficient.
The thermostat valve plate is stuck in the "open" ("offen") position, for example due to deposits; flow to the radiator remains constantly open. The engine stays in cold-start operation for too long. Briefly, this should not cause any damage. Nevertheless, the thermostat should be replaced as soon as possible.

B. The temperature gauge needle swings fully to the right despite a correct coolant level, and the coolant warning lamp flashes. The radiator and the lower radiator hose remain cold.

The thermostat valve plate is stuck in the "closed" ("zu") position (for example, due to a faulty or leaking thermostat element). Under no circumstances continue driving, otherwise the engine may suffer serious damage from overheating! Replace the thermostat immediately.

Note: If the engine overheats on the road due to a faulty thermostat, either a tow truck or removing the thermostat on site can help. If you decide to remove the thermostat, wait until the coolant temperature has dropped. During this time you can obtain containers for draining the coolant.