The ambient temperature sensor is not directly linked to the engine control unit but to the instrument cluster. The value of the ambient temperature is sent to the engine control unit via CAN bus. In the event of a fault, the instrument cluster sends a characteristic value which the DME interprets as a fault.
If a fault is entered, a substitute value is calculated for the ambient temperature from the intake-air temperature. The ambient temperature is required for many functions in the engine control unit that contain temperature-dependent parameters.