Powertrain, generic · Transmission

P085B Gear Shift Control Module B Communication Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

Gear shift control modules A and B are selector modules of shift-by-wire systems, such as Audi's selector lever J587 or Stellantis' rotary shift module. The codes cover a dedicated link to the gearbox module and the module's plausibility. If the selected range cannot be transferred reliably, the gearbox holds its range, goes to P at standstill or into limp mode. Modules on CAN report lost communication as U0103 or invalid data as U0404 instead; GM uses its own codes. P085B means the link to module B stays at a low level, typically a short to ground or a sender that no longer releases the line. No maker documents it.

Likely causes

Communication line shorted to ground
Power or ground of the selector module
Software of selector or gearbox module
Selector module faulty

Symptoms

  • Selector or gearbox warning in the instrument cluster
  • Range cannot be selected or only with delay, car stays in P/N
  • Gearbox limp mode

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

electronic shift moduletransmission control moduleserial data linemodule wiring

Related codes

P085AP085CP085EP0861U0103

References: law.cornell.edu, static.nhtsa.gov, static.nhtsa.gov.