P0864TCM Communication Circuit Range/Performance
- 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. P0864 means signals arrive but in the wrong format: Subaru sets it after 60 s without a valid frame while driving above 500 rpm and 10.9 V, MIL after two trips. The service procedure checks shifting, then interference from aftermarket radios with bad grounds, then replaces the gearbox module. GM's 2005 2.8 and 3.6 V6 report it with U0101 when gearbox messages are missing for 2.5 s.
- What you may notice
- Check engine lamp after two trips
- Shift problems or limp mode possible
- Often no noticeable effect
- Likely causes
- LikelyGearbox module faulty or its software
- PossibleInterference from retrofitted radio or phone with poor ground
- PossibleLoose contact in wire or connector between the modules
- RareLow supply voltage
- 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.