The ECM/PCM "A" has stopped transmitting on the vehicle data bus, so other control units no longer receive its messages. As this is the engine controller itself, the engine usually will not start or run while the message is missing. Typical causes are loss of power or ground at the module, an open or shorted CAN/LIN bus wire, a connector fault, or an internal module failure.
High severity. Can I keep driving?Do not keep driving: the engine may stall or not restart, and while driving you may lose driver assistance systems. If it happens on the road, get to safety and call a tow truck.
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Most likely causes
- ECM unpowered (fuse, ground, supply wire)
- CAN wiring to ECM broken
- ECM internally failed
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It can trigger limp mode.
What each character of U0100 means
- U: Communication network
- 0: Generic code: same on every make
- 1: Subsystem depends on the code range
- 00: Specific fault in that system
Frequently asked questions
Can I keep driving with code U0100?
Do not keep driving: the engine may stall or not restart, and while driving you may lose driver assistance systems. If it happens on the road, get to safety and call a tow truck.
What causes code U0100?
The most common causes are: eCM unpowered (fuse, ground, supply wire); cAN wiring to ECM broken; eCM internally failed.
Related codes
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.