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P0851 OBD-II Trouble Code: Park / Neutral Switch Input Circuit Low

Check Engine Light

P0851 code definition

The code P0851 indicates that the voltage reading from the park/neutral switch input circuit was too low compared to the manufacturer’s specifications for the current driving conditions. This code is only found in vehicles with both automatic transmissions and four-wheel-drive capability.

What the P0851 code means

In order to calculate engine RPM, engine timing, fuel delivery, and many other essential operations, the PCM uses data from a wide variety of sensors that make up the park/neutral switch input circuit. If the data from these sensors indicate that the transmission shifter range is falling below the set parameters, this code will be stored.

What causes the P0851 code?

The P0851 code is most often caused by a transfer case range sensor that is not properly adjusted, particularly if there were recent repairs performed. However, there may be other causes, including:

  • A defective range sensor.
  • Corrosion within the sensor connector.
  • Any open, corroded, shorted, or damaged electrical components, including wiring and connectors.
  • Improperly installed sensor retaining bolts.

What are the symptoms of the P0851 code?

The Check Engine lamp should illuminate after this code is stored in the PCM. The vehicle owner may observe several other symptoms, including:

  • Lack of ability to engage four-wheel drive.
  • Very harsh shifting.
  • Lack of ability to shift gears.
  • Reduced fuel efficiency.

How does a mechanic diagnose the P0851 code?

To begin diagnosing the P0851 code, a mechanic must first verify the code with the OBD-II scanner. Then, they will visually inspect all the electrical components, and replace any faulty, open, shorted, or damaged elements.

After this repair, and any others, they should clear the PCM and retest the system. This step ensures that the mechanic knows precisely which repair fixed the issue, and ensures that no unnecessary repairs are performed. If the code returns after this step, the mechanic will move on to testing other components of the park/neutral switch until the fault is discovered.

Common mistakes when diagnosing the P0851 code

Often, after a vehicle has undergone recent transmission repairs, the range sensor will be left improperly adjusted. This means that all that is needed is a minor readjustment to fix this issue. Many technicians report that parts are mistakenly blamed and replaced when this was the case.

How serious is the P0851 code?

Failure to address this code when it arises could lead to dangerous driving conditions, or a vehicle that will not operate at all. The P0851 code directly relates to the safe and proper operation of the vehicle; therefore, it is very important that this issue be repaired immediately.

What repairs can fix the P0851 code?

There are a few ways that a technician can address a P0851 code detection. They include:

  • Readjustment of an improperly adjusted transfer case range sensor.
  • Replacement of a defective range sensor.
  • Replacement of a corroded sensor connector.
  • Repairing any open, corroded, shorted, or damaged electrical components, including wiring and connectors.
  • Reinstalling an improperly installed sensor retaining bolts.

Need help with a P0851 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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