U0100

U0100 is a standardized diagnostic trouble code (DTC) used in OBD-II systems to indicate a loss of communication with the vehicle’s ECU, typically the ECM/PCM or another control module on the vehicle’s data network. The code is generated when the powertrain control module does not receive expected messages from another module within the designated time frame, signaling a network communication fault rather than a single sensor or actuator issue.

Common causes include faults on the data network or its power supply, such as bad wiring or connectors, damaged CAN/LIN bus lines, blown fuses, or poor ground and battery voltage. A failing ECM/PCM or another module on the network, software incompatibilities, or the presence of aftermarket devices interfering with the bus can also trigger U0100. Weather, moisture, or corrosion at connections can contribute as well.

Symptoms may be varied and can include a illuminated check engine light, loss of communication between modules, or intermittent performance issues. Other DTCs may appear alongside U0100, especially those referencing specific modules that fail to communicate.

Diagnosis typically begins with a scan to confirm the DTC and review any accompanying codes. Technicians inspect electrical power and grounding to the ECM/PCM, examine fuse status, and test the CAN/bus lines for continuity and proper signal levels. Visual inspection of connectors and wiring for damage, corrosion, or moisture is common. Depending on findings, tests may include module-specific communication checks, oscilloscopic analysis of the data bus, and, if necessary, software updates or module replacement.

Because the exact module referenced by U0100 can vary by vehicle make and model, OEM-specific definitions and repair procedures should be consulted. After repairs, codes are cleared and communication should be verified across all affected modules.