What does U0100 mean?
The control module detected a condition described as “Lost communication with engine control module”. The code identifies the affected area, but it does not by itself prove which component has failed. The code describes the detected condition, not a guaranteed failed part. A reliable diagnosis also considers related codes, freeze-frame data, live values and the actual symptoms.
Possible causes
- Low voltage or voltage spike
- Loose connector, ground fault or damaged CAN wiring
- A control module fails or restarts
- Aftermarket equipment interferes with the data bus
Common symptoms
- Several warning lights at the same time
- Control modules intermittently unavailable
- Starting problems or function failures
- Faults occur sporadically
Sensible next checks
- Stabilise and verify system voltage first
- Compare all stored communication codes and timestamps
- Inspect connectors, grounds and CAN wiring
- Diagnose the individual unavailable control module directly
Why does it return after clearing?
Clearing the code only removes the stored entry. If the underlying condition remains, the control module will set the code again after the next relevant monitor cycle. Save the data first, diagnose the cause, repair it and only then clear the code.