U0100

Lost Communication With ECM/PCM

🔴 CRITICAL — STOP DRIVING NETWORK · CAN Bus

A control module on the CAN bus cannot communicate with the Engine Control Module.

What You'll Notice
  • Multiple warning lights across clusters — ABS, SRS, traction control all on simultaneously
  • Instrument cluster gauges dropping to zero
  • Engine may not start
  • Transmission stuck in limp mode
Probable Causes
  • CAN bus wiring fault — chafed, corroded, or accidentally cut harness
  • Failed PCM / ECM module
  • Bad PCM ground or power supply circuit
  • Corroded battery terminals causing intermittent voltage drop on CAN network
Direct Repair Steps
  1. Check battery voltage and terminal connections first — low voltage causes false U-codes
  2. Verify PCM power and ground circuits with a DVOM (should read battery voltage and <0.1 Ω ground resistance)
  3. Inspect main engine harness connectors at PCM for corrosion or pushed-back pins
  4. Use advanced scan tool to check CAN bus health; look for bus-off errors indicating a module pulling the bus low
Vehicles Affected
chevy ford gmc dodge jeep bmw honda toyota
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