U0100 – Lost communication with engine control module

U0100 means communication with engine or powertrain control module ECM/PCM A has been lost. Another module can report the missing communication without itself being faulty.

Network · Communication between control modules

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Meaning and affected system

Vehicle controllers exchange operating information over a network. Missing expected messages can produce a communication code. Diagnosis needs to establish which module recorded U0100 and whether it is a current fault or a stored past event.

Possible causes

Open or shorted network wiring and poor connector contact are possibilities.
Module power supplies and ground connections also need vehicle-specific checks.
A failed controller is possible but is not established by U0100 alone.

Possible symptoms

Multiple warnings or an inaccessible control module may be noticed.
Starting problems or restricted functions are possible, depending on the network failure.

Useful diagnostic considerations

Save a complete module scan and identify which controllers still communicate.
Distinguish current faults from historical entries and document recent battery or wiring work.
Have network layout, supplies and connections checked to manufacturer guidance before replacing a module.

What this code does not prove

U0100 is not proof of a faulty engine controller. One shared wiring or connection problem can generate several communication messages.

Look up another fault code

Sources and interpretation

This explanation was editorially cross-checked against the technical sources below. Manufacturer documents concern particular vehicle applications; their thresholds and repair procedures must not be applied universally.

OBD-II code data: OBDex (CC0 1.0) · 196fddffb6ef

Editorial source review: 16 October 2026.

The causes listed are possibilities without a likelihood ranking. This page cannot replace diagnosis of the vehicle. A code alone confirms neither a failed component nor that continuing to drive is safe. The actual condition, warnings, handbook and applicable manufacturer diagnostics determine the next step.