Uxxxx · Network
U0100
Lost Communication with ECM/PCM "A"
CriticalNetwork ECU
What it means
Another module on the CAN bus has lost communication with the Engine Control Module. The ECM may be unpowered, off the bus, or fully failed.
Common causes
Listed in rough order of frequency. Verify with vehicle-specific service info before parts replacement.
- 01ECM has no power (blown fuse, failed relay, harness fault)
- 02ECM internal failure
- 03CAN bus wiring damage between modules
- 04Recent module replacement without coding / programming
- 05Vehicle in transport / theft-recovery mode
Affected systems
ECUs and subsystems that report or are affected by this code.
CAN busEngine Control ModuleAffected reporting module
Get vehicle-specific diagnosis for U0100
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Ask the forums about U0100Critical-severity code
This code typically prevents the vehicle from starting, running normally, or operating safely. Diagnose carefully before clearing — clearing without fixing the root cause often masks deeper problems.