P085B

Gear Shift Control Module B Communication Circuit Low

Transmission Generic OBD-II Medium

What it means

P085B means the communication circuit to gear shift control module 'B' is reading low, so the module and the rest of the transmission network can't exchange data reliably. The engine still starts, but the transmission may hold a gear, shift poorly, or drop into limp mode. It usually points to wiring, a corroded or wet connector, or a power/ground problem at the module.

Medium

Can I keep driving? The engine starts, but if the shift control module drops offline the transmission may enter limp mode or refuse to change gear — have it diagnosed soon.

Common symptoms

What you'll notice

Check-engine light on
Transmission stuck in one gear or slow to shift
Limp mode / reduced-power warning
Warning lights for transmission or shift-by-wire faults

Common causes

Most likely first

Wiring fault, short, or open in the communication circuitLikely
Corroded connector or moisture in the gear-shift module plugCommon
Poor ground or supply voltage to the shift control moduleCommon
Faulty or mismatched gear shift control module 'B'Less often

How to diagnose & fix

Work through it in order

Visually inspect the module connectors and wiring for damage, corrosion, or water intrusion; dry and repair as needed.
Check the module's power and ground circuits for the correct voltage.
Test the communication circuit for continuity and a short-to-ground causing the low signal.
Confirm the fitted module matches the vehicle's specification (after any prior repairs).
Replace the shift control module only if wiring, power, and ground test good. Clear and re-test.

FAQ

Quick answers

Is it safe to drive with P085B?+

If the transmission still shifts, drive gently, but a communication loss can trigger limp mode without warning, so get it checked promptly.

Can I just clear P085B?+

It returns until the wiring, connector, or module fault is fixed — communication faults rarely clear on their own.

Related codes

Often seen with P085B

P085AGear Shift Control Module B Communication P085CGear Shift Control Module B Communication P0866TCM Communication Circuit High P0700Transmission Control System (MIL Request)

Read & clear P085B yourself

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✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX