P0865: TCM Communication Circuit Low

What does P0865 mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine lamp after two trips
• Gearbox faults not passed to the engine module
• Shift problems possible

Common causes

• High Line between the modules shorted to ground
• Medium Gearbox module faulty, holding the line low
• Medium Poor contact at engine or gearbox module connector
• Low Pull-up in the engine module faulty

Likelihood is a general estimate across vehicles; your make and model may differ.