What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code and applies to many OBD-II vehicles (1996-newer). This may include, but is not limited to, vehicles from Toyota, Honda, Acura, Lexus, Geo, General Motors, etc. Although generic, the exact repair steps may vary depending on the year, make, model, and powertrain configuration.
If your vehicle has stored a P0812 code, it means that the Powertrain Control Module (PCM) has detected a malfunction in the reverse input signal circuit.
The Transmission Control Module (TCM) can be a standalone module or part of the PCM. Like the TCM, the reverse light sensor for the transmission can be a single component or it can be part of the neutral safety switch. Vehicle applications with a manual transmission typically use a separate reverse position sensor, and automatic transmissions use the neutral safety switch.
The transmission controller calculates data from the vehicle speed sensor and wheel speed sensors to determine if the vehicle is moving or not; and in which direction. The PCM also performs periodic self-tests to ensure the system circuit voltage is within parameters.
If the PCM detects that the vehicle is moving in a reverse direction without the appropriate reverse sensor signal, a P0812 code may be stored and a Malfunction Indicator Lamp (MIL) illuminated. Several ignition cycles (with a failure) may be required for the MIL to light.
How severe is this DTC?
A stored P0812 code indicates that a serious electrical problem or a type of mechanical failure has occurred. In any case, the conditions that contributed to the storage of a code of this nature should be rectified as soon as possible.
What are some of the symptoms of the code?
Symptoms of a P0812 trouble code may include:
Reverse lights are inoperative
Reverse lights are continuously on
No symptoms may be apparent
What are some of the common causes of the code?
Causes
of this code may include:
Faulty reverse position sensor
Faulty neutral safety switch
Shorted wiring for the reverse position sensor
Faulty PCM or programming error
What are the P0812 troubleshooting steps?
A diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of vehicle-specific diagnostic information will be needed to diagnose a P0812 code.
You can use your vehicle information source to locate a Technical Service Bulletin (TSB) that matches the vehicle’s year, make, and model; as well as engine size, stored codes, and presented symptoms. If you find one, it could provide useful diagnostic information.
Use the scanner (connected to the vehicle’s diagnostic connector) to retrieve all stored codes and relevant freeze frame data. It’s a good idea to record this information before clearing the codes, then test drive the vehicle until the PCM enters readiness mode or the code resets.
If the PCM enters readiness mode at that point, the code is intermittent and may be much more difficult to diagnose. If this is the case, the conditions that contributed to the code being stored may need to worsen before an accurate diagnosis can be made.
If the code immediately resets, the next step in your diagnosis will require you to consult your vehicle information source for diagnostic flowcharts, connector pinout diagrams, 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 a reference voltage (transmission in reverse) or a ground (transmission not in reverse).
Step 2
If the system circuits are functional, use the DVOM to test the appropriate reverse position sensor. Replace sensors that do not test within system parameters.
Reverse position sensor codes are most often attributed to a sensor failure