P0812: Reverse Input Circuit

What it means

A fault was detected in the reverse input circuit. The code does not establish which other vehicle systems use that input.

At a glance

• Usually lights the check-engine light

How it works

The reverse input circuit carries a signal indicating a reverse-related input to the transmission control system. The module monitors that circuit for an electrical fault and sets P0812 when the input does not behave as expected. The code does not specify which vehicle functions, beyond the reverse input, use the signal.

Common causes

Most likely first.

1. Open circuit in reverse input wiring
2. Short to ground or power in the input circuit
3. Loose or corroded connector
4. Faulty reverse switch or sensor
5. Control module circuit fault

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 transmission live data for reverse input status and related codes to see whether the signal is missing or unstable.
2. With the vehicle secured, compare the displayed reverse input status with actual selector position without relying on the dashboard indication alone.
3. Inspect the reverse switch or sensor connector and harness for loose pins, corrosion, chafing, or other visible damage.
4. Test the reverse input wiring for opens and shorts to ground or power, and check switch or sensor operation against the manufacturer's procedure.
5. If the input device and wiring pass, check the module connector, power, grounds, and input circuit before considering a module fault.

Common fixes

• Repair an open, grounded, or power-shorted reverse input wire.
• Clean or repair loose or corroded connector terminals.
• Replace a faulty reverse switch or sensor after confirming it fails its test.
• Repair a module circuit fault only after the input device and wiring pass inspection.

Can I keep driving?

Use extra care when selecting reverse, and do not rely on an uncertain gear indication or reverse response. If the vehicle engages the wrong gear or cannot select gears predictably, stop driving until the input fault is checked.