U0100 Code: Lost Communication with ECM/PCM — Causes & Fix
U0100 means a module has lost communication with the engine control module (ECM/PCM) — it can no longer exchange data with the engine computer.
- Applies to
- All OBD-II vehicles (any make or model)
- Est. repair cost
- $50 – $900
- Time to fix
- 1–4 hours
What it means
Causes include ECM power/ground loss, CAN wiring faults to the ECM, or a failed ECM. U0100 is a generic OBD-II network/communication ("U") code defined in the SAE J2012 standard, so it applies to virtually any modern OBD-II vehicle regardless of make. These faults usually point to the vehicle's data bus (often CAN) or a module that has dropped off the network. Fix the wiring/module fault, then clear the code with a scan tool.
Symptoms
- Check engine light + other warnings
- No-start or stalling
- Gauges/features not working
How to fix it
- 1
Check ECM power and grounds
1. Verify the ECM has power and good grounds. 2. Inspect the CAN wiring to the ECM.
- 2
Diagnose the ECM/network
1. Scan for whether the ECM responds at all. 2. Repair wiring or replace the ECM, then clear the code.
Frequently asked questions
Can U0100 cause a no-start?
Yes — if other modules can't talk to the engine computer (or it has lost power), the car may not start. Check ECM power, grounds, and CAN wiring.
Last updated July 19, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.