There is no diagnostic function on this system.You need either the dedicated Bosch test rig or check the system manually from the ecu located behind the battery.First item I would check is the over voltage relay located behind battery.This has 1 or 2 fuses in the top.It is also very easy to check sensor resistance on these. 2 fronts and 1 in the rear axle.Are there any cold starting or running issues,as this relay also powers the KE jetronic system?