P0851
Park/Neutral Switch Input Circuit Low
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.
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.
Related codes
Often seen with P0851
Read & clear P0851 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX