U0100 — Lost communication with ECM/PCM

What this code generally means, its most common causes, and how serious it is.

TYPE: GENERICSYSTEM: NETWORK COMMUNICATION

What does the code U0100 mean?

A modern vehicle's electronic modules talk to each other over an internal network (CAN bus). This U (network) code means another module reported losing communication with the engine control module (ECM/PCM) — not necessarily that the ECM itself has failed.

Common possible causes

A wiring problem on the communication network (CAN bus) — one of the most common causes.
A blown fuse powering the engine control module — cuts its power without it being an internal failure.
A corroded ground connection — can affect several modules' communication at once.
A failing engine control module itself — less common, but possible.

General severity

Serious — get it checked promptly

This code can come with several other warning lights on at once and unpredictable engine behavior, since it affects communication with the engine's central module. Worth taking seriously, especially if the car runs poorly or several lights appear together.

This is still a general severity for this code, not a diagnostic for your car. Only a real read of your codes, with your mileage and vehicle history, can tell what's really going on.

Get a diagnostic for YOUR car

Connect a Bluetooth OBD2 reader and read your vehicle's real codes with Meca — free, no appointment, with causes ranked for your actual situation.

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Meca doesn't replace an in-person check by a professional. For any serious sign (a flashing light, a major loss of power, a fuel smell), see a mechanic promptly.

Frequently asked questions

Several warning lights came on at once — is that related?

Yes, that's a common symptom of this kind of code — a network communication loss can affect several modules at once, not just the engine.

Is my engine control module dead?

Not necessarily — this code signals a communication loss, which can come from wiring, a fuse, or a ground connection, not only a failure of the module itself.

Can I keep driving with this code?

That isn't recommended without a check, especially if the car runs poorly — better to get it checked promptly given how central this module is.