Code Definition

P0851 — Park / Neutral Switch Input Circuit Low

System: Powertrain  |  Type: Generic (OBD-II Standard)  |  Make: Generic

About P0851

P0851 is a generic (obd-ii standard) powertrain diagnostic trouble code. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0851 is stored, the module has detected a signal or supply voltage lower than the expected operating range.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

Because P0851 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The P0851 fault code is set when the vehicle's control module detects a low voltage condition in the Park/Neutral Position (PNP) switch input circuit. This code indicates that the voltage signal received from the PNP switch is below the expected range, suggesting an electrical problem such as a short to ground. The PNP switch communicates to the control module whether the vehicle is in park or neutral, and a low voltage signal may lead to incorrect readings, affecting the vehicle's starting and operational characteristics.

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