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P0812 OBD-II Trouble Code: Reverse Input Circuit

Check Engine Light

P0812 code definition

The P0812 trouble code identifies a reverse input circuit problem.

What the P0812 code means

This code is triggered when the PCM (Powertrain Control Module) has discovered a problem with the reverse input circuit. When the driver puts their vehicle in reverse, they do so by using the shifter, which is then energized. The PCM monitors this process to ensure the voltage from this switch falls within an acceptable range predetermined by the manufacturer.

When the PCM detects an irregular amount of voltage or that there was resistance from the reverse input circuit, the P0812 code will be stored.

What causes the P0812 code?

There are three main causes that usually result in a P0812 code being stored:

  • The reverse input switch may be defective
  • Wiring that facilitates the use of the reverse input switch may be short or open
  • This wiring could also be damaged In very rare scenarios, the PCM may actually be to blame and storing codes wrongfully, but it’s more likely to be one of the above (the first is actually the most common culprit).

What are the symptoms of the P0812 code?

This code will almost always coincide with the Check Engine light coming on. However, some models are designed to only activate this light once the problem has been detected at least two or three times. In some cases, the backup lamps may also quit working.

How does a mechanic diagnose the P0812 code?

In order to diagnose the problem behind this code, the mechanic will need to:

  • Use an OBD-II scanner to download all the stored codes from the PCM
  • Capture this data on a freeze frame and record all relevant information like the time and frequency with which they occurred
  • Diagnose and repair any other problems that were identified through stored codes on the PCM
  • Carry out an examination of the wiring and connectors to look for loose connectors or corrosion
  • Check for voltage on the input switch
  • Check to see if the fuse needs to be replaced
  • Put the vehicle in reverse and check the voltage on the output side of the input switch
  • Confirm resistance and continuity between the reverse input switch and the PCM Before testing any circuitry, the mechanic should disconnect any related control modules. Doing so will prevent potential damage to the PCM and other costly repairs.

Common mistakes when diagnosing the P0812 code

As we mentioned above, one of the symptoms of this code is that the backup lamp may quit working. This is usually because of a short in the wiring, though it could also have to do with a faulty reverse input switch or bad connectors. However, a hasty mechanic may immediately assume that the problem has to do with the lamp bulbs.

How serious is the P0812 code?

There is nothing inherently dangerous about the P0812 code. That being said, if it results in the backup lamps burning out, the driver will need to get around with them. That can be dangerous as other drivers won’t have forewarning when the automobile is backing up. Getting caught with this problem could earn the owner a ticket, too, one that will probably cost a lot more than just going in for repairs.

What repairs can fix the P0812 code?

Repairing a malfunctioning reverse input circuit will usually involve some combination of the following:

Additional comments for consideration regarding the P0812 code

When carrying out repairs, the manufacturer’s diagram for the specific make and model should be used to make sense of the circuits that need to be tested. If the vehicle has an automatic transmission, this switch is usually part of a neutral safety switch or manual lever position sensor. With manual transmissions, the switch may be threaded by the gear shifter to the transmission housing.

Need help with a P0812 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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