P2181 Thermostat Stuck Open

P2181

Thermostat stuck open

Diagnostic conditions
- Vehicle speed > 15 km/h (9 mph)
- Engine speed 1,000 ... 5,000 rpm
- Air mass throughput under the aforementioned conditions: 3.0 kg
- Coolant temperature when engine starts < 60 °C (140 °F)
- Ambient temperature -6.8 °C (19.8 °F) ... 60 °C (140 °F)

NOTE: In this context, please also observe the function description.

Possible fault causes
- Thermostat faulty (stuck open)
- Coolant temperature sensor faulty
- Tiptronic vehicles only: coolant shutoff valve open (mechanical fault, lack of vacuum...)

Function description

NOTE: The diagnostic conditions can best be established on a (roller-type) dynamometer.

Start of diagnosis after engine starting under the following conditions:
- Engine not hot
- Ambient temperature in a suitable range
- Normal driving
- Adequate air mass throughput

Diagnosis procedure:
- The air throughput is used to estimate the amount of heat liberated during combustion of the fuel mixture.
- If, under the conditions described above, the coolant temperature does not reach a certain value after a certain amount of fuel mixture has passed through the engine, it is assumed that the thermostat is stuck in open position.
- Since a stuck temperature signal also produces a similar fault symptom, the fault "engine temperature sensor" might also be entered. The sensor must be checked in this case.
- In Tiptronic vehicles, an open coolant shutoff valve can also lead to this fault entry, as the engine heat is transferred to the transmission via the transmission cooling.