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Error Code P0812 is defined as Reverse Input Circuit. Error Code P0812 is a generic trouble code, meaning it applies to all vehicles with the OBD-II system, especially those made from 1996 up to present. This includes vehicles from, but not limited to, Acura, General Motors, Geo, Honda, Lexus, Toyota, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model and powertrain configuration.
If the vehicle registers the Error Code P0812, that means PCM (powertrain control module) has determined a malfunction in the reverse input signal circuit.
The TCM (transmission control module) is sometimes a standalone unit. In recent years, however, it’s been commonly integrated into the single housing of the ECM (engine control module). This component is now referred to as the PCM. Like the TCM, the transmission’s reverse sensor may be a single component or part of a neutral safety switch. Vehicles equipped with manual transmission uses separate reverse position sensor, while vehicles with automatic transmissions use the neutral safety switch.
Both the vehicle speed sensor and wheel speed sensors send data to the TCM. The latter calculates these data to determine whether the vehicle is in motion or not, and in what direction. The PCM also periodically runs self-tests to make sure the system circuit voltage is within the parameters.
If the PCM determines that the vehicle is moving in the reverse direction without the right reverse sensor signal, then this error code will be stored, and simultaneously activate the Check Engine light. In some cases, multiple ignition cycles (with a failure) may be needed to activate the Check Engine light.
- Faulty reverse lamps
- Reverse lamp continuously activated
In some cases, the vehicle may show no symptoms at all.
- A defective reverse position sensor
- Shorted wiring for the reverse position sensor
- Faulty neutral safety switch
- Defective or programming error in PCM
How to Check
You will also need a diagnostic scanner, DVOM (digital volt/ohmmeter), and a source of vehicle diagnostic information.
Use the vehicle’s information to find the TSB (technical service bulletin) that matches the specifics of the vehicle (make, model, engine size, symptoms, and codes stored). This could help for a more thorough diagnosis.
Use the scanner (connected to the vehicle diagnostic connector) to retrieve all stored codes and pertinent freeze frame data. It is a good idea to write this information down before clearing the codes then test-drive the vehicle until the PCM either enters readiness mode, or the code is reset.
If the PCM enters readiness mode at this time, the code is intermittent and may be much more difficult to diagnose. If this is the case, the conditions which contributed to the code being stored may need to worsen before an accurate diagnosis can be made.
If the code is immediately reset, the next step of your diagnosis will require that you search your vehicle information source for diagnostic flow-charts, connector pin-out charts, connector face views, and component testing procedures/specifications.
Next, using the DVOM, test the voltage, ground, and signal at the reverse position sensor or neutral safety switch. The reverse position sensor input and output circuits usually consist of either reference voltage (transmission in reverse) or ground (transmission not in reverse).
If the system circuits are working properly, then test the right reverse position sensor using the DVOM. Any sensors that do not test within system parameters must be replaced.
Note that reverse position sensor codes are most often attributed to sensor failure.
How to Fix
- Replacement of reverse input switch
- Repair or replacement of faulty or corroded connectors or wiring
This code indicates a serious electrical problem or mechanical failure, so it must be addressed as soon as possible.