Powertrain, generic · Transmission

P0865 TCM Communication Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

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

Symptoms

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

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–1,500
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

transmission control moduleengine control moduleserial data linemodule wiring

Related codes

P0863P0864P0866U0101P0700

References: saabwisonline.com, saabwisonline.com, law.cornell.edu.