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.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: hard
What we see in the workshop
ON ALFA ROMEO
Lost communication with the ECM is a familiar Alfa complaint and is more often a connector, earth strap or body control module fault than a failed ECU. We diagnose the network before anyone replaces a control unit.
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
Common causes
Ordered by how often each turns out to be the cause.
- ECM unpowered (fuse, ground, supply wire)Common
- CAN wiring to ECM brokenPossible
- ECM internally failedLess likely
Parts involved
- ECM
- CAN Bus Wiring
U0100 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.