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P0851 is a diagnostic trouble code (DTC) for "Park / Neutral Switch Input Circuit Low". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.
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.
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.
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:
The Check Engine lamp should illuminate after this code is stored in the PCM. The vehicle owner may observe several other symptoms, including:
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.
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.
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.
There are a few ways that a technician can address a P0851 code detection. They include:
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