Technical Description of OBD-II Trouble Code
Reverse Input Circuit
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to many OBD-II vehicles (1996-newer). This includes vehicles from Toyota, Honda, Acura, Lexus, Geo, General Motors, and others. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
If your vehicle has stored a code P0812, it means that the powertrain control module (PCM) has detected a malfunction in the reverse input signal circuit.
The transmission control module (TCM) may be a standalone module or part of the PCM. Similar to the TCM, the reverse sensor for the transmission may be a single component or part of the neutral safety switch. Vehicles with manual transmissions typically use a separate reverse position sensor, while automatic transmissions use the neutral safety switch.
The transmission controller uses data from the vehicle speed sensor and wheel speed sensors to determine if the vehicle is in motion and in which direction. The PCM also performs periodic self-tests to ensure that the system circuit voltage is within the specified parameters.
If the PCM detects that the vehicle is moving in reverse without receiving the appropriate reverse sensor signal, it may store a code P0812 and illuminate the malfunction indicator lamp (MIL). Multiple ignition cycles with a failure may be required for the MIL to illuminate.
What is the severity of this DTC?
A stored code P0812 indicates that there is either a serious electrical issue or some type of mechanical failure. In any case, it is important to rectify the conditions that led to the storage of this code as quickly as possible.
What are some of the symptoms of the code?
Symptoms of a P0812 trouble code may include:
What are some of the common causes of the code?
The causes for this code may include:
What are some troubleshooting steps for P0812?
To diagnose a code P0812, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of vehicle-specific diagnostic information.
You can use your source of vehicle information to find a technical service bulletin (TSB) that matches the year, make, and model of your vehicle, as well as the engine size, stored codes, and exhibited symptoms. If you find a relevant TSB, it may provide helpful diagnostic information.
Use the scanner connected to the vehicle diagnostic connector to retrieve all stored codes and relevant freeze frame data. It is recommended to write down this information before clearing the codes and 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 point, the code is intermittent and may be more difficult to diagnose. In such cases, the conditions that led to the storage of the code may need to worsen before an accurate diagnosis can be made.
If the code is immediately reset, the next step in your diagnosis will involve searching your vehicle information source for diagnostic flowcharts, connector pin-out charts, connector face views, and component testing procedures/specifications.
Step 1
Use the DVOM to test the voltage, ground, and signal at the reverse position sensor or neutral safety switch. The input and output circuits of the reverse position sensor typically consist of either reference voltage (when the transmission is in reverse) or ground (when the transmission is not in reverse).
Step 2
If the system circuits are functioning properly, use the DVOM to test the appropriate reverse position sensor. Replace any sensors that do not test within the specified system parameters.
