OBD-II code · Network
U0100 Lost Communication with ECM/PCM "A"
The ECM/PCM "A" has stopped transmitting on the vehicle data bus, so other control units no longer receive its messages. As this is the engine controller itself, the engine usually will not start or run while the message is missing. Typical causes are loss of power or ground at the module, an open or shorted CAN/LIN bus wire, a connector fault, or an internal module failure.
Is it safe to drive with U0100?
The car may go into limp mode (reduced power). Get it diagnosed before a long trip.
Common causes
- Most likelyECM unpowered (fuse, ground, supply wire)
- PossibleCAN wiring to ECM broken
- Less likelyECM internally failed
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Repair at a glance
- Difficulty
- Hard
- DIY friendly
- Best left to a mechanic
- Typical labour
- 0.5–8 h
- Emissions test
- Usually unaffected
- Parts involved
- Ecm, Can Bus Wiring
Related codes
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.