What are the common causes of U0100?

Shorted or open data bus wiring to the PCM. Faulty PCM or failing internal network interface. Poor PCM power or ground connections. Failed module interrupting the network. Corrosion or water damage in connectors.

Verify PCM power, grounds, and battery voltage. Check the data bus wiring between modules for shorts or opens. Measure bus resistance and inspect connectors. Isolate network modules to find the interfering device. Replace the PCM or repair wiring after bus checks confirm the cause.

U0100

Lost Communication With ECM/PCM A

System: Network communication (U code)
Lamp: Check Engine Light
Category: Network Communications

What it means

U0100 means Lost Communication With ECM/PCM A.

Check these first

Shorted or open data bus wiring to the PCM
Faulty PCM or failing internal network interface
Poor PCM power or ground connections
Failed module interrupting the network
Corrosion or water damage in connectors

What to do next

Verify PCM power, grounds, and battery voltage
Check the data bus wiring between modules for shorts or opens
Measure bus resistance and inspect connectors
Isolate network modules to find the interfering device
Replace the PCM or repair wiring after bus checks confirm the cause

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Updated

Generic SAE J2012 definition. Last updated 2026-08-30.