General Description of Outside Temperature Monitoring (plausibility check)

For this monitor the engine coolant temperature is compared to the ambient temperature after allowing sufficient time for the engine to cool (e.g.engine-off time>= 12 h).

A malfunction will be set if the temperature difference between the ambient and the engine coolant exceeds a threshold.

Furthermore, the diagnostic strategy includes check for certain conditions that could cause a "false" MIL illumination.

For example, after an overnight soak at places with high ambient temperatures during the day and low temperatures at night, the ambient and coolant temperatures could decrease down to e.g. 20°F. After the sun rises, the solar load on the exterior of the vehicle will cause the ambient temperature (as measured by the outside temperature sensor) to increase much quicker than the engine coolant temperature. If the diagnostic would compare the engine coolant temperature with this ambient temperature under these circumstances it would lead to an incorrect conclusion.

Therefore, BMW introduced two surety checks within the diagnostic to avoid false fault codes and MIL illuminations.

After a soak of at least 12 hours, if the difference between the coolant and ambient temperature exceeds a delta of >3°C, a malfunction is suspected.

Surety check 1:

A comparison between the ambient temperature at engine shut-off (before soak) and engine restart (after soak) is made. If there is a temperature delta more than 3°C, the sensor is not stuck.

and

Surety check 2:

A comparison of the ambient temperature at engine restart is made to the ambient temperature after a certain amount of accumulated time driven above a certain speed (necessary to compensate filter effects of the ambient temperature). If surety check 2 verifies that there is a temperature delta of more than 3 °C the sensor is not stuck.

These two checks ensure that the outside temperature sensor is not stuck even if there was an unacceptable difference between the ambient and the engine cooling system temperatures.