U0100 means Lost Communication With ECM/PCM A. Another module on the vehicle network has stopped hearing from the engine computer. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What U0100 means
Modern vehicles have many control modules that share information over a network, usually called the CAN bus. Codes starting with U mean a module has stopped hearing another one, and voltage and processor codes mean a module is not getting clean power or has an internal fault. Low battery voltage, poor grounds, blown fuses and damaged wiring cause many of these codes, so test the battery and charging system first.
U0100 symptoms
- No-start or stalling
- Several warning lights and dead gauges
- Transmission stuck in one gear
Common causes of U0100
- Engine computer without power or ground (blown fuse, bad relay, corroded ground)
- Damaged network wiring or connectors
- Low battery voltage
- Failed engine computer
How to diagnose U0100
- Check battery voltage and the engine computer fuses and relays
- Check module power and grounds
- Inspect network wiring and connectors near the module
- Check whether a scan tool can communicate with the engine computer at all
Can you drive with U0100?
Usually not; a no-start or stall is common. Diagnose before driving.
U0100 tips
U0100 is set by another module, not by the engine computer itself. If no module can talk to the engine computer, start with its fuses, relays and grounds; if only one module reports U0100, look at the network wiring to that module.
Will U0100 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing U0100 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| U0101 | Lost Communication With TCM |
| U0073 | Control Module Communication Bus A Off |
| P0562 | System Voltage Low |
Guides that help
When it is more than a sensor
Voltage and network codes come from wiring, the battery, the charging system or a module, not from engine or transmission wear. If a lost-communication code has left the transmission in limp mode, rule out the module and wiring before deciding the unit has failed, and see our transmissions only if testing confirms internal damage.
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