Wiring fault on the communication circuit, such as a short to voltage or chafed insulation; Corroded, loose or damaged connector at the gear shift module or the control unit; Low battery voltage or a poor power or earth supply to the gear shift module; Faulty gear shift control module B (internal failure); Faulty powertrain or transmission control module driver circuit; Outdated control unit software where the manufacturer has identified it as a known cause.

P085C

Gear Shift Control Module B Communication Circuit High.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
8: AreaTransmission
5C: FaultThe specific fault in that area

What it means

P085C means the powertrain control module saw a high voltage condition on the communication circuit between it and gear shift control module B. This is the module or circuit that handles gear selection requests, such as an electronic selector or shift-by-wire unit. The signal was held high, out of range or missing, so the control unit could not trust the selector information. How the circuit is arranged and which module counts as B depends on the vehicle.

Symptoms

Gearbox warning light or malfunction message on the dash
Engine management light on
Gear selector not responding, or gear position not shown correctly
Gearbox stuck in one gear or limp mode
Car will not start, or will not engage drive or reverse
Other communication fault codes stored at the same time

Possible causes

Wiring fault on the communication circuit, such as a short to voltage or chafed insulationlikely
Corroded, loose or damaged connector at the gear shift module or the control unitlikely
Low battery voltage or a poor power or earth supply to the gear shift modulepossible
Faulty gear shift control module B (internal failure)possible
Faulty powertrain or transmission control module driver circuitrare
Outdated control unit software where the manufacturer has identified it as a known causerare

How to diagnose

Read all stored and pending codes and note any other communication or power supply faults, then record the freeze frame data.
Check battery condition and charging voltage, as low voltage can cause false communication faults.
Inspect the wiring and connectors at the gear shift module and the control unit for damage, water ingress, corrosion, pushed-back pins and chafing.
Check the fuses and the power and earth supplies to the gear shift module against the wiring diagram.
With the ignition off and the battery disconnected as the procedure allows, test the communication circuit for continuity, and for short to voltage or to earth.
With the ignition on, measure the voltage on the communication circuit and compare it with the manufacturer's specification.
Check for technical service bulletins or software updates for your model and engine that relate to this code.
If the wiring and supplies are good, test or substitute the gear shift module following the manufacturer's procedure, and confirm the cause before replacing any module.

How to fix

Repair or replace damaged wiring on the communication circuit, using proper soldered or sealed repairs.
Clean, re-seat or replace corroded or damaged connectors and terminals.
Restore a good power and earth supply to the gear shift module, and replace the battery if it is weak.
Replace the gear shift control module B if it is confirmed faulty, and code or adapt it to the car as required.
Fit a manufacturer software update only if a bulletin identifies outdated software as the cause.

Can I keep driving?

Medium severity

The car may still be driveable, but gear selection may be unreliable or the gearbox may drop into limp mode. Get it checked soon, and avoid driving if you cannot reliably select gears or park the car safely.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.