Gear Shift Module Communication Circuit

Possible causes

• CommonLow battery voltage or a discharged battery, dropping modules off the bus
• CommonDamaged, chafed or corroded bus wiring
• CommonCorroded or water-ingressed connector at the module
• CommonBlown fuse or corroded earth on the module supply
• PossibleFailed module
• PossibleModule not coded to the vehicle after replacement
• PossibleAftermarket electrical work spliced into the bus

What you might notice

• Transmission stuck in one gear
• Gear display blank or wrong
• Several warning lights at once
• Scan tool cannot see the module
• Check engine light on

What to check, in order

1. Check battery voltage and state of charge before anything else. A weak battery drops modules off the bus and sets communication codes with no wiring fault at all - the same false lead that dominates the U-code family.
2. Check the fuses and earths for the gear shift module.
3. Note how many communication codes are stored. Several at once points at battery, earth or bus wiring rather than at a failed module.
4. Inspect the module connector for corrosion and water.
5. Ask what was done to the car recently. Aftermarket alarms, trackers and stereos spliced into the bus are a frequent cause.
6. Do not order a module until power, earth, bus wiring and battery condition are all eliminated. A replacement usually needs coding as well.