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OBD-II · Network and communication (U)

U0100

Lost communication with engine control module (ECM/PCM A)

What it means

Another module (often the gearbox, ABS or instrument cluster) has stopped receiving messages from the engine control unit.

Symptoms

  • Engine may stall, not start or run in limp mode
  • Several warning lights and gauges dropping out
  • Gearbox stuck in one gear

Likely causes, most likely first

Likely causes, most likely first
No.Cause
01weak 12V battery or poor battery terminals/earth
02blown fuse or failed relay feeding the engine ECU
03corroded ECU connector or water ingress into the ECU (common where the ECU sits under the scuttle/plenum)
04damaged CAN wiring between ECU and the rest of the network
05failed engine ECU

What to check, step by step

  1. 01Check 12V battery, terminals and main earth straps
  2. 02Check ECU fuses and main relay; confirm power and ground at the ECU connector
  3. 03Check whether the ECU answers a scan tool at all
  4. 04Inspect the ECU and its connector for water or corrosion, including a blocked scuttle drain
  5. 05Test CAN wiring from the ECU to the gateway or next module