U0100: Lost Communication With ECM/PCM "A"
U0100 means Lost Communication With ECM/PCM "A". Another module (often the transmission, ABS or instrument cluster) has stopped receiving messages from the engine control module.
Lost communication with the engine computer often goes with a no-start. Check the engine computer's fuses, power and grounds before suspecting the module itself.
Can I keep driving? Limit driving and diagnose promptly. Continued driving can cause damage, stalling or limp mode.
Common symptoms
- Several warning lights at once
- Modules or features not working
- Possible no-start or limp mode, depending on the module
Possible causes
- Low battery voltage or poor ground
- Blown fuse or lost power to the module
- Damaged CAN bus wiring or connector
- Failed module
- Aftermarket accessories tapped into the network
How to diagnose U0100
- Check battery voltage and grounds first.
- Scan all modules to see which ones do not respond.
- Check power and ground to the non-responding module.
- Measure resistance between CAN-H and CAN-L at the diagnostic connector with the battery disconnected; many high-speed CAN networks read about 60 Ω.
- Inspect wiring and connectors along the bus.
What the code means, digit by digit
Generic codes have the same definition on every OBD-II vehicle (petrol cars from model year 1996 in the US, 2001 in the EU; diesels later). Manufacturers can add their own enhanced codes; those are not listed here.
Symptoms linked to U0100
Related codes
- U0101 Lost Communication With TCM
- U0121 Lost Communication With Anti-Lock Brake System (ABS) Control Module
- U0140 Lost Communication With Body Control Module
- U0155 Lost Communication With Instrument Panel Cluster (IPC) Control Module
- U0073 Control Module Communication Bus "A" Off
- U0001 High Speed CAN Communication Bus
General guidance only. Fault codes describe what the engine computer saw, not which part failed: confirm with testing before replacing parts, and follow the vehicle's service information.