What does code U0100 mean?

U0100 — Lost communication with Engine Control Module (ECM/PCM) over CAN bus. Dash may show errors, no tach, engine may not start. Belongs to the network.lostcomm system. Severity: critical.

What are the common causes of U0100?

Break in CAN bus between ECM and rest of vehicle; CAN shorted to ground — entire bus goes silent; ECM lost power — blown fuse, bad relay; Bad ECM ground — typical after engine bay washing; Failed ECM — less common

Can I drive with code U0100?

Not recommended. Continued driving can damage the engine, transmission, or catalyst. Use a tow truck or take the shortest possible route to a workshop.

How do I diagnose U0100?

Read the code and freeze-frame with a scanner, check related-system errors, inspect the components listed (wiring, connectors, sensors). See the "How to diagnose" section above for the full step-by-step.

How do I clear code U0100?

The code can be cleared via an OBD-II scanner (e.g. ELM327 + the Car Diagnostics app): plug the adapter into the OBD port, go to Errors → Clear. If the underlying cause isn't fixed, the code will return within a few drive cycles.

OBD-II Code U0100 — Car Diagnostics Platform

What it means

Common causes

Break in CAN bus between ECM and rest of vehicle
CAN shorted to ground — entire bus goes silent
ECM lost power — blown fuse, bad relay
Bad ECM ground — typical after engine bay washing
Failed ECM — less common
CAN shorted to +12V through corroded connectors

How to diagnose

01
Read all stored and pending codes with an OBD-II adapter — get the full picture, not just the one code.
02
Capture freeze-frame data (RPM, coolant temperature, engine load) at the exact moment the code triggered.
03
Inspect the named subsystem with our app's AI explanation — get vehicle-specific causes for your make, model and year.

Decode U0100 on your actual car

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