P0851 Code: Park/Neutral Switch Input Circuit Low

What the P0851 code means

The park/neutral (P/N) position switch tells the computer whether the shift lever is in park or neutral. It allows the engine to start and it adjusts idle speed when you engage a gear.

It is a simple switch, so the computer sees either a high or a low voltage depending on its position. The P0851 code is set when that signal sits below its minimum. A low signal usually comes from a signal wire shorted to ground or from a missing reference voltage.

When the signal fails, the engine may not start, or it may start while a gear is engaged, which is dangerous, and idle speed can drop when you shift into drive. A shift cable that is out of adjustment produces the same symptoms. The transmission range sensor is tested the same way, so check it as well.

To confirm the diagnosis, unplug the component with the key on and watch what happens to the scan tool reading or to the code. If the value jumps to the opposite extreme, the wiring is fine and the component is the part pulling the signal to ground. If nothing changes, look for the short in the wire, starting where the harness rubs against the engine, the chassis or the exhaust.

P0851 code symptoms

• Check engine light on.
• Engine will not start, or idles wrong.
• Engine does not start in park or neutral.
• Idle speed drops when you shift into gear.

Likely causes of the P0851 code

1. Signal wire shorted to ground where the harness rubs against the engine, the chassis or a bracket.
2. Reference supply shorted. If several sensors fail at once, suspect the shared reference.
3. Damaged switch with an internal short.
4. Connector with water or corrosion, which creates a path to ground.

How to diagnose the P0851 code step by step

1. Read the switch value on the scan tool and compare it with what you expect.
2. Inspect the connector for corrosion, water or loose pins.
3. Check power and ground with the key on: the reference should be correct and the ground voltage drop should be under 0.1 V.
4. Measure the signal with the connector plugged in and see whether it changes with the lever position.
5. Do a wiggle test on the harness with the scan tool in graph mode.
6. Check continuity of each wire to the computer (under 1 ohm).
7. Check the shift cable adjustment.
8. Check the P/N state on the scan tool in every lever position.

How to fix the P0851 code

• Repair the connector or the wiring.
• Replace the switch if the tests confirm it is bad.
• Correct the real condition if there is one.
• Clear the codes and verify with a road test.

Technical reference data

• The switch or sensor is powered from the computer. Check the values in the service manual.
• Reference values are only a guide, so look up the manufacturer data for your vehicle.
• Wire continuity should be under 1 ohm, and ground voltage drop under 0.1 V.

Brand-specific notes

The letter assignment and the location of the component change from one manufacturer to another, so check the manual for your vehicle before you start testing.

Because a P0851 code can stop the engine from starting, confirm the shift cable adjustment early. It is a free check and it rules out a mechanical cause that looks exactly like an electrical one.

Frequently asked questions about P0851