Lost Communication with ECM/PCM 'A'
Another module on the network can't talk to the engine PCM. CAN bus or PCM power problem.
What it means (plain English)
Modern vehicles have a network (CAN bus) that lets modules talk to each other. U0100 means some other module — TCM, BCM, ABS, instrument cluster — tried to communicate with the engine PCM and got no response. Either the PCM is offline (no power, no ground, dead), or the bus wiring between them is broken / shorted. Symptoms range from no-start to a dash full of warning lights.
What the computer is actually seeing
A module on the bus expected a periodic CAN message from the PCM and didn't receive it within the timeout window.
What a healthy reading looks like
CAN-H and CAN-L should idle at 2.5 V each, switching ~1 V apart during traffic. Bus resistance pin-to-pin should be ~60 Ω (two 120 Ω terminators in parallel).
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 1Try to communicate with PCM directly via scan tool. If you can — bus is fine, problem is elsewhere.
- 2Check PCM fuses and relays.
- 3Measure CAN bus resistance KOEO at the DLC — should be ~60 Ω.
- 4Scope CAN-H / CAN-L for healthy traffic.
- 5Disconnect aftermarket add-ons (alarm, remote start, hardwired dashcam) and retest.
Common causes
- PCM power or ground fault (blown fuse, corroded ground)
- PCM failure
- Broken / shorted CAN bus wire
- Failed CAN terminator resistor
- Aftermarket accessory tapping bus wires badly
- Water intrusion at a connector
Typical repair cost
$50 (fuse) to $1,500 (PCM + programming).
Related codes
Frequently asked questions
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