P0851

Park/Neutral Switch Input Circuit Low

Q: Is it safe to drive with P0851?

Once the engine is running the car drives fine, but the starter interlock and shift logic depend on this signal, so it's worth fixing promptly.

Q: Can I just clear P0851?

It returns until the switch, its wiring, or connector is repaired.

Transmission Generic OBD-II Medium

What it means

P0851 means the park/neutral switch input is reading abnormally low, so the computer can't reliably tell whether the transmission is in Park or Neutral. That signal drives the starter safety interlock and shift logic, so the engine may refuse to crank and the display may show the wrong gear. It's usually a faulty range switch, wiring, or connector.

Medium

Can I keep driving? The car drives normally, but a bad park/neutral input can stop the engine from cranking (the safety interlock) or disturb shift logic — fix it soon.

Real-world scan data

Seen in

scans in the past year

Found in

<0.1%

of scans

<0.1%

of scans that found faults

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

Common symptoms

What you'll notice

Check-engine light on
Engine won't crank in Park or Neutral (starter interlock)
No reverse/backup lights, or wrong gear shown on the display
Delayed engagement or erratic shift behaviour

Common causes

Most likely first

Faulty park/neutral (transmission range) switchLikely
Corroded or damaged wiring in the input circuitCommon
Loose or disconnected connector at the switchCommon
Faulty transmission control module (TCM)Less often

How to diagnose & fix

Work through it in order

Inspect the range/park-neutral switch connector for corrosion, damage, or a loose fit.
Check the switch's signal wiring for a short-to-ground or open causing the low reading.
Test the switch outputs in each gear position and compare to spec.
Adjust or replace the park/neutral switch if the signal is wrong.
Replace the TCM only after wiring and switch causes are ruled out. Clear and re-test.

FAQ

Quick answers

Is it safe to drive with P0851?+

Once the engine is running the car drives fine, but the starter interlock and shift logic depend on this signal, so it's worth fixing promptly.

Can I just clear P0851?+

It returns until the switch, its wiring, or connector is repaired.

Read & clear P0851 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX