P0866TCM Communication Circuit High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- The TCM communication circuit is a direct link between gearbox and engine module outside the CAN bus. Subaru's 2005 WRX, sold as Saab 9-2X, uses a single 'AT diagnosis' wire: about once a second the gearbox module sends a frame with a start pulse of 40 ms low and 20 ms high, carrying among other things its request for the MIL. On CAN cars, loss of the gearbox module sets U0101 or U0402 instead. P0866 means the line stays high for more than 3 s with no start pulse, MIL after two trips. Over 10 V at the engine module pin points to a short to voltage; a voltage switching between 1 and 4 V is a healthy signal. A gearbox module without power sends nothing, so the pull-up keeps the line high and the gearbox runs in a fixed gear.
- What you may notice
- Check engine lamp after two trips
- Gearbox in limp mode with fixed gear if its module has no power
- Gearbox faults not passed to the engine module
- Likely causes
- LikelyGearbox module without power or ground, or faulty
- PossibleLine between the modules open
- PossibleLine shorted to voltage
- PossiblePoor contact at a module connector
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €80 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.