In order to make your transmission function, a number of sensors, control modules, connectors and wires are used communicate data about the vehicle to the engine control unit. If an unusually high-voltage signal is detected on this circuit, the ECU will save a P0866 DTC.
How Serious is the Code?
If the necessary sensors and control modules cannot communicate with the engine control unit, a number of serious drivability issues can occur. So a P0866 diagnostic trouble code is pretty serious.
Symptoms of a P0866 include:
- Delayed or harsh shifts
- Erratic shifting behavior
- Limp mode
- Damage to the wiring and connectors on the transmission wiring harness
- Transmission control unit fault
- Engine control unit failure
- Software issue
How to Diagnose the Code?
When diagnosing a P0866 OBDII trouble code, the wiring and connectors on the transmission wiring harness will need to be inspected for signs of damage and corrosion. The manufacturer’s database of TSB’s should be checked for known fixes and software updates. If nothing has been found by this point, the TCM and ECU should be considered.
What Repairs Will Fix P0866?
- 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
P0865 – TCM Communication Circuit Low