U0100 — Lost Communication With ECM/PCM A

U0100 means another module stopped receiving CAN bus messages from the engine control module. Wiring, connectors and power supply faults cause this far more often than a dead ECM.

Cost range: $100–$2,000. Severity: high. Safe to drive? No. Modules that can't communicate can disable the engine or safety systems mid-drive.

Most common causes of U0100

1. CAN bus wiring fault — Open, short or corrosion in the twisted pair. Check bus resistance at the DLC (should be ~60 ohms). ($150–$600)
2. Loss of power or ground to the ECM — Blown fuse or corroded ground strap. ($50–$250)
3. Low battery voltage — Bus errors are a classic symptom of a dying battery. ($150–$400)
4. Failed ECM/PCM — Confirm only after wiring and power checks pass. ($800–$2,000)

Symptoms drivers report with U0100

• Multiple warning lights
• No start
• Gauges dead or erratic
• Several modules reporting U-codes