Powertrain, generic · Transmission

P0863 TCM Communication Circuit

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. P0863 is the general fault of this link without telling range, low or high apart. No maker documents it on its own. Without the link the engine module receives no gearbox status, and a MIL request from the gearbox gets lost.

Likely causes

Wire or connector between engine and gearbox module open or corroded
Gearbox module without power or ground
Gearbox module faulty
Input of the engine module faulty

Symptoms

  • Check engine lamp on
  • Gearbox warning or limp mode possible
  • Gearbox faults not passed to the engine module

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€100–1,500
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

P0864P0865P0866U0101U0402P0700

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