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P0851 GMC Code - Park/Neutral Position Switch Circuit Low Voltage

Quick Answer

The P0851 diagnostic trouble code (DTC) for GMC vehicles indicates that there is a low voltage condition detected in the Park/Neutral Position (PNP) switch circuit. Common causes include Faulty PNP switch, Wiring or connector issues in the PNP switch circuit, Corrosion or damage to the electrical connections. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means
P0851 GMC indicates Park/Neutral Position Switch Circuit Low Voltage.
Common causes
Common causes include Faulty PNP switch, Wiring or connector issues in the PNP switch circuit, Corrosion or damage to the electrical connections.
Severity
High - diagnose this code as soon as possible.
Can I drive?
Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
What is this?
Table of contents Created and updated by William Orellana on Sep 10, 2024
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Code P0851 GMC Description

The P0851 diagnostic trouble code (DTC) for GMC vehicles indicates that there is a low voltage condition detected in the Park/Neutral Position (PNP) switch circuit. The PNP switch, also known as the transmission range sensor, is responsible for informing the vehicle's computer about the gear selection (Park, Neutral, Drive, etc.). In this case, the low voltage indicates that there is a problem with the electrical signal being sent from the PNP switch to the engine control module (ECM).

OBDII Code P0851 GMC - Park/Neutral Position Switch Circuit Low Voltage
P0851 GMC Code - Park/Neutral Position Switch Circuit Low Voltage

Common Causes of P0851 GMC

  1. Faulty PNP switch
  2. Wiring or connector issues in the PNP switch circuit
  3. Corrosion or damage to the electrical connections
  4. ECM software or hardware issues
  5. Transmission fluid leaking onto the PNP switch

Symptoms of P0851 GMC

  1. Difficulty starting the vehicle
  2. Incorrect gear selection display on the dashboard
  3. Harsh shifting or erratic transmission behavior
  4. Engine stalling when shifting gears
  5. Check Engine Light illuminated on the dashboard

How to Fix P0851 GMC

  1. Begin by diagnosing the PNP switch circuit using a multimeter to check for voltage levels and continuity.
  2. Inspect the wiring harness and connectors for any signs of damage, corrosion, or loose connections. Repair or replace as necessary.
  3. Test the PNP switch itself for proper operation. If found faulty, replace with a new one.
  4. Clear the DTC from the ECM memory using an OBD-II scanner and perform a test drive to verify that the issue has been resolved.
  5. If the problem persists, further diagnostics may be required to identify any underlying issues with the transmission or ECM.

Cost to Fix P0851 GMC

The cost of repairing a P0851 code for a GMC vehicle can vary depending on the specific model and year of the vehicle, as well as the extent of the repairs needed. In general, replacing a faulty PNP switch can cost anywhere from $150 to $300, including parts and labor. Additional costs may be incurred if there are wiring or connector issues that need to be addressed. It is recommended to consult with a reputable auto repair shop for a more accurate estimate based on the specific circumstances of your vehicle. Labor rates can range from $80 to $150 per hour, with rates potentially higher in certain locations or at dealerships.

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Information for Specific GMC Models

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

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