P0851 Park/Neutral Switch Input Circuit Low

P0851 indicates the park or neutral switch input circuit is low. The module sees a grounded, missing, or below-range park/neutral signal.

Code details

Generic OBD-II · P0

System: Transmission
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace starter components before confirming the low park/neutral input circuit.

Symptoms of P0851

Harsh, delayed, or slipping shifts, or being stuck in one gear.
Check-engine light, sometimes with limp-home mode.
Flaring RPM or a shudder under load.

Common causes of P0851

Low or degraded transmission fluid.
A failing speed sensor, shift solenoid, or pressure-control circuit.
Wiring, connector, or internal hydraulic/mechanical faults.

Useful checks before replacing parts

Measure park/neutral switch signal voltage, feed, ground, and continuity while selecting park, reverse, neutral, and drive.
Inspect sensor or switch adjustment, terminal tension, rubbed wiring, short-to-ground faults, and corrosion near the transmission.
Compare live park/neutral data with starter relay command and range sensor data to confirm the low input's effect.

Related OBD-II codes

These nearby transmission entries can help compare wording and pin down the fault area.