The transmission control module communication circuit consist of a number of wires, sensors and modules that collect data about a number of vehicle functions. Without this information the ECU cannot, determine the proper shift strategy or engine setup. If there’s an issue with this data stream, a P0865 DTC will be set.
How Serious is the Code?
The P0865 diagnostic trouble code is extremely serious, because it basically means the engine control computer cannot communicate with the transmission and its modules. This can result in a number of drivability issues, and could potentially leave you with a nonworking transmission.
Symptoms of a P0865 include:
- Erratic, harsh or delayed shifts
- Limp mode
- Damaged wiring and/or connectors on the transmission wiring harness
- Transmission control module failure
- ECU failure
- Programming faults
How to Diagnose the Code?
Diagnosing a P0865 OBDII trouble code should begin with a thorough check of the wiring and connectors on the transmission wiring harness. If no issues are found, the manufacturer’s TSB database should be checked for known software updates and issues. Should nothing be found, the transmission control module and engine control unit should be tested.
What Repairs Will Fix P0865?
- Repair or replace damaged wiring on the transmission wiring harness
- Replace or repair broken/corroded connectors
- Replace transmission control module
- Replace engine control unit
- Update TCM/ECU software
- P0864 – TCM Communication Circuit Range/Performance
P0866 – TCM Communication Circuit High