If it is the bcu at fault, it doesn't log it internal faults so diagnostic tools cannot tell you it is at fault.
It is only when you have tested everything else, or you start to get other random faults related to bcu controlled systems eg hazard warning lights turning on for no apparent reason or failures in the security system that you may conclude that the bcu is faulty.
In my case all intermittent faults, it started with failure of an internal switch supplying power to xyz switch, then hazard lights turning on randomly and finally problems with the security system.
Replaced bcu with new one programmed and synced to system with nanocom with help from Rick at Callrova and all problems solved.