Powertrain, generic · Transmission

P0812 Reverse Input Circuit

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

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.

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

Symptoms

  • Back-up lamps never or always on
  • Reversing camera or parking aid does not switch on
  • Usually no check engine lamp

Repair estimate

DifficultyEasyDIY possible
Parts and labour€30–200
Time0.5–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reverse switchswitch wiringengine control moduletransmission control module

Related codes

P0813P0801P0705P0850P0914

References: law.cornell.edu, patents.google.com, gsi.ext.gm.com.