Damaged wiring harness or connector at the gear shift control module B.; Corrosion or water intrusion in the communication circuit connectors.; Faulty gear shift control module B.; Short to power in the communication circuit.; Poor ground connection for the module.; PCM internal fault affecting the communication circuit.

DTC codes / P08xx · Transmission / P085C

P085CGear Shift Control Module B Communication Circuit High

Q: What does code P085C mean?

Gear Shift Control Module B Communication Circuit High. P085C indicates that the powertrain control module (PCM) has detected a high voltage condition on the communication circuit for the gear shift control module B. This module is responsible for executing gear changes, and the high voltage suggests a short to power or an open circuit in the wiring or connectors.

P085C indicates that the powertrain control module (PCM) has detected a high voltage condition on the communication circuit for the gear shift control module B. This module is responsible for executing gear changes, and the high voltage suggests a short to power or an open circuit in the wiring or connectors.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaTransmission

Symptoms you may notice

Illumination of the check engine light (malfunction indicator lamp).
Transmission may enter limp mode, limiting gear selection.
Harsh or delayed gear shifts.
Inability to shift into certain gears or engage drive/reverse.
Often no symptoms besides the warning light.

Common causes, most likely first

Damaged wiring harness or connector at the gear shift control module B.
Corrosion or water intrusion in the communication circuit connectors.
Faulty gear shift control module B.
Short to power in the communication circuit.
Poor ground connection for the module.
PCM internal fault affecting the communication circuit.

How to diagnose and fix it

Use a scan tool to verify the code and check for related codes in other modules.
Visually inspect the wiring and connectors at the gear shift control module B for damage or corrosion.
Perform a voltage drop test on the communication circuit to check for high resistance.
Check for short to power by measuring voltage on the communication circuit with the module disconnected.
Inspect and clean ground connections for the module.
If wiring is intact, test the gear shift control module B with a known-good unit.
Repair or replace faulty wiring, connectors, or the module as needed, then clear codes and verify.

Common Missteps in P085C Diagnosis

A high voltage on the communication circuit often results from a short to power, which can be caused by a pinched wire against a 12-volt source.
Technicians sometimes replace the gear shift control module B prematurely when the real issue is a corroded connector that can be cleaned or repaired.
The code can be set by a loose or damaged ground connection, leading to erratic voltage readings that mimic a module failure.
Always check for technical service bulletins (TSBs) for the specific vehicle, as some models have known harness routing issues that cause this code.
Intermittent faults may not appear during static testing; a wiggle test of the harness while monitoring live data can reveal the problem.

Can I keep driving with P085C?

If the vehicle is in limp mode or shifting erratically, avoid driving at high speeds or under heavy load and seek service soon. If only the warning light is on and the transmission behaves normally, you can drive cautiously to a repair shop, but do not ignore the code as it may worsen.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.