U0100: OBD-II code U0100 flags loss of communication with the engine control module — other modules can no longer see the PCM on the network bus.

Symptoms

• Check engine light and multiple other warning lamps illuminate together as modules lose the PCM's data.
• The engine may run in limp mode with reduced power, or refuse to start entirely if the PCM is fully offline.
• Transmission, ABS, and stability control also degrade because they depend on live PCM torque and status data.

Common causes

• A failed PCM — the module itself is offline due to internal failure or corrupted firmware.
• A CAN bus fault (broken wire or short) between the PCM and the rest of the network.
• Loss of power or ground at the PCM — blown fuse, corroded ground strap, or damaged main harness.

What to do?

Confirm the PCM has 12 V power and a solid ground at its connector. Check the fuse feeding the PCM. Measure CAN-H and CAN-L continuity to the PCM connector. If power, ground, and bus are all good and the PCM won't wake up, replace it and reprogram to the vehicle.