P0863TCM Communication Circuit
- 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. P0863 is the general fault of this link without telling range, low or high apart. No maker documents it on its own. Without the link the engine module receives no gearbox status, and a MIL request from the gearbox gets lost.
- What you may notice
- Check engine lamp on
- Gearbox warning or limp mode possible
- Gearbox faults not passed to the engine module
- Likely causes
- LikelyWire or connector between engine and gearbox module open or corroded
- PossibleGearbox module without power or ground
- PossibleGearbox module faulty
- RareInput of the engine module faulty
- 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: €100 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.