Powertrain, generic · Transmission

P0866 TCM Communication Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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. P0866 means the line stays high for more than 3 s with no start pulse, MIL after two trips. Over 10 V at the engine module pin points to a short to voltage; a voltage switching between 1 and 4 V is a healthy signal. A gearbox module without power sends nothing, so the pull-up keeps the line high and the gearbox runs in a fixed gear.

Likely causes

Gearbox module without power or ground, or faulty
Line between the modules open
Line shorted to voltage
Poor contact at a module connector

Symptoms

  • Check engine lamp after two trips
  • Gearbox in limp mode with fixed gear if its module has no power
  • Gearbox faults not passed to the engine module

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

P0863P0864P0865U0101P0700

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