Lost Communication With ECM/PCM "B"

Possible causes

• CommonLow battery voltage or a discharged battery, dropping modules off the bus
• CommonBlown fuse or corroded earth on the supply to the engine control module (ECM/PCM "B")
• CommonDamaged, chafed or corroded bus wiring
• CommonCorroded or water-ingressed connector at the module
• PossibleFailed module
• PossibleAnother module faulting and dragging the whole bus down
• PossibleModule not coded or configured after a previous repair or replacement
• PossibleAftermarket electrical work spliced into the bus - alarms, trackers, tow bar wiring, stereos

What you might notice

• Limp-home mode or reduced power
• Transmission stuck in one gear or refusing to shift
• Engine will not start or cuts out
• Several warning lights at once
• Scan tool cannot see the module at all
• Fault comes and goes

What to check, in order

1. Check battery voltage and state of charge before anything else. A weak or failing battery drops modules off the bus and sets lost-communication codes with no wiring fault at all - this is the single most common false lead in this whole family.
2. Check whether this code is current or historic. Lost-communication codes are routinely set by a flat battery, a jump start or a battery disconnection, and a historic code straight after one of those events usually records the event rather than a fault.
3. Check the fuses and earth points for the engine control module (ECM/PCM "B") before anything else electrical. A blown fuse is cheap to find and accounts for a large share of these codes.
4. Note how many U-codes are stored. Several at once points at battery, earth or bus wiring, not at several modules failing simultaneously - and a gateway code among them is very likely the root cause.
5. Ask what was done to the car recently. Aftermarket alarms, trackers, tow bar wiring and stereos are often spliced straight into the bus and are a frequent cause of communication faults that appear weeks later.
6. Inspect the module connector for corrosion and water ingress, and wiggle-test the harness at chafe points with the scan tool connected.
7. Do not replace the module until power, earth, bus wiring and battery condition have all been eliminated. A replacement usually needs coding to the vehicle as well, so a wrong guess is expensive twice.