P0812 Reverse Input Circuit

P0812 points to a reverse input circuit fault. The controller is not receiving a valid signal that indicates reverse selection or reverse-related input state.

Code details

Generic OBD-II · P0

System: Transmission
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not diagnose reverse engagement mechanically until the reverse input signal has been confirmed.

Symptoms of P0812

Harsh, delayed, or slipping shifts, or being stuck in one gear.
Check-engine light, sometimes with limp-home mode.
Flaring RPM or a shudder under load.

Common causes of P0812

Low or degraded transmission fluid.
A failing speed sensor, shift solenoid, or pressure-control circuit.
Wiring, connector, or internal hydraulic/mechanical faults.

Useful checks before replacing parts

Compare reverse input data with actual gear selection and check whether reverse lamps, backup camera, or shift logic respond correctly.
Inspect range switch, reverse switch, shifter wiring, connector condition, fuse feed, ground, and module input voltage.
Check whether the fault appears only in reverse, during shifter movement, after rain, or after transmission or console service.

Related OBD-II codes

These nearby transmission entries can help compare wording and pin down the fault area.