What does code U0100 mean?
Symptoms seen in the workshop
- Several warning lights on at once (ABS, gearbox, engine)
- U0100 stored in other modules, not in the engine ECU
- Gearbox in limp mode
- If the ECU is not working: the engine does not start
Common causes of code U0100
- ECU disconnected to read or write it on the bench or in boot mode (code stored while it was out)
- No power to the ECU or low battery during cranking
- ECU from another vehicle or variant, with a different CAN configuration
- ECU in boot mode or without software after an interrupted flash
- Damaged ECU CAN wiring, ground or connector
Fix by ECU remapping
In most cases nothing needs to be reprogrammed:
1. After bench or boot work: with the ECU refitted and running, clear the codes in every module. If U0100 does not come back, it was only the record of the time the ECU was out.
2. If it comes back (ECU coding): check that the ECU has the software, variant and CAN coding the other modules expect, especially if it comes from another vehicle, and recover it if it was left in boot mode.
It is not solved with DTC OFF in the engine ECU: the code is stored by the other modules, and hiding it there would cover up a real communication problem.
⚠️ Important: Disabling the DTC without the correct calibration of the related system (ECU coding) is not a valid fix. Both the calibration maps and the diagnostic mask have to be modified.
