The P0866 trouble code indicates a high voltage condition in the transmission control module (TCM) communication circuit.
P0866 TCM Communication Circuit High
Code symptoms P0866.
1. Check Engine Light (CEL) illuminated on the dashboard.
2. Difficulty or inability to shift gears smoothly.
3. Harsh shifting or erratic transmission behavior.
4. Transmission may go into limp mode, limiting the vehicle’s speed.
5. Other transmission-related issues may be present.
Causes of the code P0866.
1. Faulty transmission control module (TCM).
2. Damaged or corroded wiring or connectors in the communication circuit.
3. Poor electrical connections or ground faults.
4. Internal transmission problems.
5. Faulty powertrain control module (PCM) or other control modules.
Code Solutions P0866.
1. Replace the faulty TCM if necessary.
2. Repair or replace damaged or corroded wiring or connectors in the communication circuit.
3. Address any issues with the PCM or other control modules as needed.
4. Clear the trouble codes from the vehicle’s computer after repairs are complete.
5. Test drive the vehicle to confirm that the issue has been resolved.