OBD-II code · Powertrain (generic)
P0866 TCM Communication Circuit High
The Transmission Control Module communication circuit reports a communication circuit high fault. Data exchange between TCM and ECM is impaired.
Is it safe to drive with P0866?
The car may go into limp mode (reduced power). Get it diagnosed before a long trip.
Common causes
- Most likelyCommunication bus fault
- PossibleTCM internal fault
- Less likelyGateway module routing CAN messages incorrectly
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
Repair at a glance
- Difficulty
- Medium
- DIY friendly
- Best left to a mechanic
- Typical labour
- 1–5 h
- Emissions test
- Usually unaffected
- Parts involved
- Tcm, Wiring Harness, Ecm
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.