P0865TCM Communication Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- 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. P0865 means the line stays low for more than 3 s, MIL after two trips. The engine module pulls the line up; the procedure measures below 1 V at the engine module pin with ignition on, checks the harness for a short to ground and then reads the gearbox codes or replaces the gearbox module.
- What you may notice
- Check engine lamp after two trips
- Gearbox faults not passed to the engine module
- Shift problems possible
- Likely causes
- LikelyLine between the modules shorted to ground
- PossibleGearbox module faulty, holding the line low
- PossiblePoor contact at engine or gearbox module connector
- RarePull-up in 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: €80 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.