P0812 concerns the input 'reverse engaged' at the engine or transmission module – the reverse or back-up lamp switch on a manual gearbox, or the R contact of a selector. Modules check it for open or stuck states and against a second source such as a gear position sensor or the direction from speed sensors. A Honda patent describes this double detection: reverse counts only when switch and gear position sensor agree; if only one reports R, it is a fault and engine speed is limited. No manufacturer document with thresholds is available to us. Back-up lamps, reversing camera or parking aids may fail; on automated manual gearboxes reverse may be blocked. CARB lets such safety and comfort inputs run without the MIL.
Medium severity. Can I keep driving?Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
Symptoms
- Back-up lamps never or always on
- Reversing camera or parking aid does not switch on
- Usually no check engine lamp
Most likely causes
- Reverse or back-up lamp switch faulty or misadjusted
- Connector loose or wire broken (e.g. after clutch work)
- Back-up lamp circuit fuse blown
- Gear position sensor reporting the wrong position
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: easy
What each character of P0812 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 8: Transmission
- 12: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0812?
Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
What causes code P0812?
The most common causes are: reverse or back-up lamp switch faulty or misadjusted; connector loose or wire broken (e.g. after clutch work); back-up lamp circuit fuse blown.
Related codes
- P0813Reverse Output Circuit
- P0801Reverse Inhibit Control Circuit/Open
- P0705Transmission Range Sensor A Circuit (PRNDL Input)
- P0850Park/Neutral Switch Input Circuit
- P0914Gear Shift Position Circuit A
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.