Powertrain, generic · Transmission

P0864 TCM Communication Circuit Range/Performance

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. 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.

Likely causes

Gearbox module faulty or its software
Interference from retrofitted radio or phone with poor ground
Loose contact in wire or connector between the modules
Low supply voltage

Symptoms

  • Check engine lamp after two trips
  • Shift problems or limp mode possible
  • Often no noticeable effect

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

P0863P0865P0866U0101U0402P0700

References: saabwisonline.com, saabwisonline.com, gsi.ext.gm.com, law.cornell.edu.