U0100

What U0100 means

U0100 is set by a module that can't reach the ECM over CAN. It's a communication fault code, not an internal ECM code. Typically caused by wiring issues, a bad connection, or a dead ECM.

Symptoms

• Multiple warning lights
• No-start or intermittent stalling
• Dashboard gauges blank or erratic
• Limp mode

Likely causes (most common first)

1. Damaged CAN bus wiring
2. Corroded or disconnected ECM connector
3. Weak battery or poor charging system
4. Failed ECM
5. Bad ground

Can I keep driving?

High — risk of damage if ignored

Typical repair cost

DIY fixes can start near $50 (parts only). Shop repairs for U0100 typically run up to $1500 depending on the underlying cause and vehicle. Start with the most common cause — Damaged CAN bus wiring — before throwing parts at it.

Symptoms that match U0100

If you got here from a search query, one of these might describe what your car is actually doing:

• Car Won't Start — Just a Clicking Sound. What to Check.
• Car Won't Start, No Click — Causes and Next Steps

Related codes

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