Body Control Unit: Description: Diagnostics

Diagnostic monitoring is included in the operating software of the BCU and is performed on a continual basis. All of the input and output signals which are relevant for diagnostics can be read out and injected using T4. Detected faults are stored in the non volatile memory (EEPROM) of the BCU.

All faults stored by the BCU are stored chronologically, with a fault counter, in a fault or information area of the memory. Faults which positively point to one or more defective components are stored in the fault area. Faults which do not positively point to a defective component are stored in the information area to provide information during troubleshooting. The BCU ignores faults detected within 32 seconds of power up or when battery voltage is less than 9 V.

A distinction is made between sporadic and static faults. Sporadic faults are those which have occurred at least once, and the cause of the fault message was no longer present at least one time. Static faults are faults which are still active at the time diagnostics are performed.

The BCU can store up to 30 faults at any one time. If further faults are detected following the 30th fault they are ignored. Faults can only be erased using T4.

Up to 16 faults stored by the BCU are also stored in those modules connected to the BCU via the P bus, i.e. the driver and passenger door modules and the sunroof module. Faults stored in the peripheral modules cannot be accessed via the diagnostic socket. The faults in the peripheral modules are used for diagnosing modules after they are removed from the vehicle.