U0100 Lost Communication With ECM/PCM "A"

U0100 sets when the reporting module stops receiving expected messages from the engine control module. The module may be dead, unpowered, or simply unable to reach the network — the code cannot tell those apart on its own.

Code details

Generic OBD-II · U0

System: Computer
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not order the engine control module on the strength of this code. Lost communication is a symptom of power, ground, wiring or network trouble far more often than it is a failed controller, and a replacement will be just as silent on a broken circuit.

Symptoms of U0100

Check-engine light, sometimes with several unrelated codes at once.
Warning lights, a no-start, or intermittent electrical faults.
Communication or module-reset problems on a scan tool.

Common causes of U0100

Low battery voltage, poor grounds, corroded connectors, or blown fuses.
Wiring or network (CAN bus) faults between modules.
A module software/programming issue, or rarely a failed controller.

Useful checks before replacing parts

Check power and ground at the engine control module first, including its fuses and any relay that feeds it, because an unpowered module is silent in exactly the same way as a failed one.
Establish whether one module is missing from the network or several are, since a whole missing segment points at bus wiring or a gateway rather than at the named controller.
Load-test the battery and check charging voltage before anything else, because low system voltage drops modules offline and sets communication codes across unrelated systems.

Related OBD-II codes

These nearby computer entries can help compare wording and pin down the fault area.