OBD-II diagnostic trouble code
U0100Stop driving
Lost Communication with ECM/PCM
Another module in the car (the one that stored this code) has lost communication with the main engine control module over the vehicle's internal network (CAN bus). This is a network-level fault, not necessarily an engine problem itself — but if the ECM genuinely stops communicating, the engine can't run.
Symptoms
- Engine may not start, or may stall and refuse to restart
- Multiple, seemingly unrelated warning lights on at once (since many systems depend on ECM data)
- Dashboard gauges may behave erratically or go blank
- Intermittent symptoms if the network fault is a loose connection rather than a full failure
Likely causes
- A blown fuse powering the ECM
- Damaged, corroded, or disconnected CAN bus wiring somewhere in the network
- A genuinely failed ECM no longer responding on the network
- A poor battery/ground connection causing intermittent module resets
How to diagnose it
- Check the ECM's power and ground connections, and its main fuse, first
- Inspect accessible CAN bus wiring for damage, especially near connectors that have been disturbed by other repair work
- A workshop scan tool can test CAN bus resistance/health directly, which is hard to do without proper equipment
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Code names are compiled from open/standardized SAE and ISO references. Explanations, symptoms, causes and fixes are original. Covers generic (P0/C0/B0/U0) codes only — manufacturer-specific codes are planned for a future update.
Frequently asked questions
Can I drive with U0100?
No — U0100 is flagged as a code where you should stop driving and get the car checked as soon as possible. Continuing to drive risks further damage or a safety issue.
How much does it cost to fix U0100?
Typical repair costs for {code} range from roughly 5 to 1800 EUR, depending on the specific cause and your car — see the fixes listed above for a breakdown.
What causes U0100?
A blown fuse powering the ECM Damaged, corroded, or disconnected CAN bus wiring somewhere in the network A genuinely failed ECM no longer responding on the network
Reviewed by Artyom SemenovAutomotive Editor · Fact-checked by Yauheni Kapliarchuk, Editor-in-Chief