Damaged, chafed or shorted wiring on the communication circuit, including a short to ground; Corroded, loose or water-damaged connector at the gear shift module or transmission control module; Low battery voltage or poor power and earth supply to the gear shift control module; Faulty gear shift control module B (selector or shifter electronics); Faulty transmission or powertrain control module affecting the network; Another network module pulling the data line down, or outdated software known to cause communication faults.

P085B

Gear Shift Control Module B Communication Circuit Low.

Generic
Powertrain
Generic code
High severity

How this code is built

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

What it means

P085B is a generic powertrain code meaning the control module has detected a low signal on the communication circuit to Gear Shift Control Module B. The module that set it saw the data line voltage held lower than expected, or messages from the gear shift module missing or corrupted. The exact layout of the circuit and which module sets the code depends on the make, model and transmission type.

Symptoms

Gearbox warning light or malfunction message on the dashboard
Gear selector not responding or gear position not shown correctly
Transmission stuck in one gear or in limp mode
Harsh, delayed or no gear shifts
Engine management light on
Car refusing to start or move, depending on the system

Possible causes

Damaged, chafed or shorted wiring on the communication circuit, including a short to groundlikely
Corroded, loose or water-damaged connector at the gear shift module or transmission control modulelikely
Low battery voltage or poor power and earth supply to the gear shift control modulepossible
Faulty gear shift control module B (selector or shifter electronics)possible
Faulty transmission or powertrain control module affecting the networkrare
Another network module pulling the data line down, or outdated software known to cause communication faultsrare

How to diagnose

Read all stored and pending codes from every module and note any other communication or low voltage codes
Review freeze frame data and check whether the fault is constant or intermittent
Test battery condition and charging voltage, as low voltage can cause communication faults
Inspect the gear shift module wiring, connectors and earths for damage, moisture, corrosion and loose pins
Check the fuses and power and earth supplies to the gear shift control module against the wiring diagram
Measure the communication line voltage and resistance as specified for the model, checking for a short to ground
Unplug suspect modules one at a time, following manufacturer procedure, to see if the line voltage recovers
Check for applicable service bulletins or software updates for the specific model and transmission

How to fix

Repair or replace damaged wiring and ensure correct routing and securing
Clean, repair or replace corroded or damaged connectors and terminals
Restore proper battery condition and repair power or earth supply faults
Replace the faulty gear shift control module and carry out any required coding or adaptation
Replace a faulty transmission or powertrain control module if it is confirmed as the cause
Apply a manufacturer software update only where a known fault with the existing calibration is confirmed

Can I keep driving?

High severity

Avoid driving until it is diagnosed, as the car may be stuck in a gear, enter limp mode or lose the ability to select gears safely. If it must be moved, only do so a short distance with great care.

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