P085C: Gear Shift Control Module B Communication Circuit High

What it means

The gear shift control module B communication circuit reports a communication circuit high condition. Shift command exchange is impaired.

At a glance

• Usually lights the check-engine light
• The engine or gearbox may go into limp mode

How it works

Gear shift control module B exchanges shift commands and status over a vehicle data bus. This code sets when the communication circuit is detected above its expected electrical level, which can result from a bus wire shorted to battery voltage or a high module output.

Common causes

Most likely first.

1. Bus wire shorted to B+
2. Module output high
3. Gateway module routing CAN messages incorrectly

Symptoms

• Check engine or transmission warning light on
• Harsh, delayed, or missing shifts
• Possible limp mode (locked in one gear)
• Slipping or shuddering during gear changes

How to diagnose it

1. Save freeze-frame data and scan all modules for network faults to establish whether the high condition is limited to module B or affects the shared bus.
2. Check whether a scan tool can communicate with gear shift control module B and review other modules' reports of lost communication.
3. Inspect the bus wiring and module B connectors for contact with a power feed, damaged insulation, corrosion, or loose pins.
4. Test the communication circuit for a short to battery voltage and check module power and ground according to the manufacturer's procedures.
5. If the wiring passes, verify gateway routing and module output before deciding a module has failed.

Common fixes

• Repair a bus wire shorted to battery voltage.
• Clean and secure a poor module B network connection.
• Correct damaged wiring or a connector fault affecting the communication circuit.
• Fix a confirmed gateway routing issue.
• Repair or replace module B only after circuit tests confirm a high module output.

Can I keep driving?

The high communication-circuit condition can interrupt shift commands and trigger a protective transmission mode. Avoid driving if shifts are erratic, gears are unavailable, or the vehicle cannot be controlled predictably.