U0100

Serious

Lost Communication With ECM/PCM A

Treat U0100 as urgent. Losing communication with the engine computer can cause a no-start or sudden stalling, and multiple systems may misbehave at once. It's not a code to drive around with — have it diagnosed before relying on the car, and don't be surprised if it won't start.

What this code means

Modern cars use a data network (CAN bus) so the computers can talk to each other. U0100 means a module lost communication with the engine/powertrain computer (ECM/PCM). This is a network or electrical fault, not a specific part failure: common causes are damaged bus wiring or a corroded connector, a blown fuse, a power/ground problem at the ECM, or the ECM itself failing. Because it depends entirely on where the communication breaks down, the repair can be anything from a cheap connector fix to a module replacement, so it must be diagnosed.

Symptoms you might notice

Multiple warning lights at once
No-start or stalling
Gauges or accessories behaving erratically
Other 'U' communication codes stored alongside it

What it costs to fix

Varies by vehicle — get it diagnosed.

The repair cost for this code depends too much on your exact vehicle and the underlying cause to quote a meaningful range. Confirm the cause first so you only pay for the fix you actually need.

Diagnose U0100 for my vehicle →

Frequently asked

Can I drive with U0100?

Not reliably. Loss of communication with the engine computer can cause stalling or a no-start and can affect several systems at once. Have it diagnosed promptly rather than driving on it.

What does U0100 mean in plain terms?

The car's computers talk over a data network, and U0100 means one of them stopped hearing from the engine/powertrain computer. It's a wiring, power, or module communication problem rather than a single mechanical part.

How much does it cost to fix U0100?

It varies widely — a wiring or connector repair is inexpensive, but a failed ECM/PCM (often needing programming) is costly. Because the cause ranges so much, it has to be diagnosed before anyone can quote it.

Is U0100 the ECM itself failing?

Sometimes, but not always. Damaged bus wiring, a corroded connector, a blown fuse, or a power/ground fault are common and far cheaper than a computer. A proper diagnosis checks those before condemning the module.

Seeing U0100 on your car? Get a diagnosis specific to your exact year, make and model.

Diagnose U0100 for my vehicle

Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.