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 status

Needs verification

Not yet verified against SAE J2012_202509 / J2012DA. Treat unverified rows as a lookup aid, not repair authority.

System: Transmission
Scope: iso-sae-controlled-p0
Standard: SAE J2012 / ISO 15031-6
Source: legacy-import

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.

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 fault area. They may share the same verification status as the source database.