P0851 Park/Neutral Position Switch Input Circuit Low

P0851 – OBD-II Trouble Code Technical Description

Trouble code P0851 indicates the Park/Neutral position switch input circuit is low.

What does the fault code mean P0851?

Trouble code P0851 indicates the Park/Neutral Position (PNP) switch input circuit is low. Also known as PRNDL on automatic transmissions, this switch controls the vehicle's gear position, including park and neutral positions. When the ECM detects that the signal from the PNP switch is below the expected level, it generates trouble code P0851.

Possible reasons

Possible causes of DTC P0851:

• Park/Neutral Position (PNP) Switch Malfunction: The switch itself may be damaged or faulty, causing its status to be read incorrectly.
• Damaged or broken wiring: The wiring connecting the PNP switch to the engine control module may be damaged or broken, resulting in a low signal level.
• Corrosion or oxidation of contacts: Buildup or corrosion on the switch contacts or connectors can cause the signal to not be read correctly and therefore cause the P0851 code to appear.
• Problems with the engine control module (PCM): A malfunction in the PCM, which controls the signal from the PNP switch, may also cause the error.
• Ground or ground problems: Insufficient grounding or ground problems in the system can lead to a low signal level and, as a result, a P0851 code.
• Problems with other vehicle systems: Certain other vehicle systems or components, such as the battery or ignition system, may interfere with the operation of the PNP switch and cause this error code to appear.

To accurately diagnose and fix the problem, it is recommended that you contact a qualified auto mechanic or service center.

What are the symptoms of a fault code? P0851?

Symptoms for DTC P0851 may include the following:

• Gear shifting problems: The vehicle may not be able to shift into the desired gears or may not shift at all. This may result in the vehicle not starting or being unable to move.
• Inability to start engine in park or neutral: If the PNP switch does not work correctly, the vehicle may not start when the ignition key is turned to the “START” position or needs to be in the “P” or “N” position.
• Malfunction of the stabilization system and/or cruise control: In some cases, the P0851 code may cause vehicle stability control or cruise control to become unavailable because these systems require gear position information.
• Error indicator on the dashboard: The Check Engine light or other LED indicators may illuminate, indicating a problem with the transmission or engine management system.
• Problems with ignition interlock: In some vehicles, the P0851 code can cause ignition interlock problems, which can make it difficult or prevent you from turning the ignition key.

If you notice one or more of these symptoms, it is recommended that you contact a qualified auto mechanic to diagnose and repair the problem.

How to diagnose a fault code P0851?

The following steps are recommended to diagnose DTC P0851:

1. Checking the LED Indicators on the Dashboard: Check for "Check Engine" lights or other LED indicators that may indicate a problem with the transmission or engine management system.
2. Using the Diagnostic Scanner: Connect the diagnostic scan tool to your vehicle's OBD-II port and read the error codes. Verify that the P0851 code is indeed present and has been recorded.
3. Visual inspection of wiring and connectors: Inspect the wiring and connectors connecting the Park/Neutral Position (PNP) switch to the engine control module. Make sure the wiring is not damaged, broken or frayed, and check the contacts for corrosion.
4. Checking the PNP switch: Check the PNP switch for proper operation. This can be done using a multimeter by measuring the resistance or voltage across its contacts at various gear positions.
5. Checking the level and condition of the transmission fluid: Check the transmission fluid level and condition as low fluid level or contaminated fluid can also cause problems with the PNP switch.
6. Additional diagnostics: If necessary, additional diagnostics may require the use of specialized equipment to check the operation of the engine control module or other transmission components.

After identifying the cause of error P0851, you should begin to eliminate it.

Diagnostic errors

When diagnosing DTC P0851, the following errors may occur:

• Lack of attention to wiring and connectors: If the wiring and connectors have not been checked carefully or any problems have not been found, it may cause the cause of the error to be missed.
• Rule out other possible causes: Focusing only on the PNP switch and not considering other possible causes, such as problems with the ECM or corrosion on the connectors, can also lead to a misdiagnosis.
• Misinterpretation of results: Incorrect interpretation of test results or measurements on the PNP switch or wiring can also lead to misdiagnosis.
• Poor diagnostics of other components: Insufficient diagnosis of other transmission system components, such as the engine control module or sensors, may result in missing additional problems that may be related to the P0851 code.
• Ignoring the level and condition of the transmission fluid: Not checking the transmission fluid level and condition may result in missing problems that may affect the operation of the PNP switch.
• Insufficient recourse to professionals: If diagnosis is carried out by a non-professional or unqualified mechanic, it may lead to incorrect conclusions and erroneous repairs.

To successfully diagnose and resolve the P0851 trouble code, it is important to contact an experienced auto mechanic or service center, especially if you encounter difficulty or uncertainty during the diagnostic process.

How serious is the fault code? P0851?

Trouble code P0851 indicates a problem with the Park/Neutral Position (PNP) switch, which is an important component of the transmission control system. Depending on how badly the switch or wiring is damaged, this problem can have different consequences. The severity of the P0851 code may be high due to the following reasons:

• Stopping a car: If the vehicle cannot be started or switched to travel mode due to a problem with the PNP switch, it may cause the vehicle to stop, which may cause inconvenience or danger on the road.
• Inability to change gears correctly: An incorrect or inoperative PNP switch position may result in the vehicle being unable to be shifted into the correct gear, which may cause loss of vehicle control.
• Inability to use stabilization and safety systems: Incorrect operation of the PNP switch may also cause some vehicle stability or safety systems to become unavailable, which may increase the risk of an accident.
• Inability to start the engine in a safe position: If the PNP switch does not work correctly, it may cause the vehicle to start in an inappropriate mode, which may result in an accident or damage to the transmission.

Based on these factors, the P0851 trouble code should be considered serious and must be diagnosed and repaired as soon as possible to ensure the safety and proper operation of the vehicle.

What repair will help eliminate the code? P0851?

Troubleshooting trouble code P0851 may involve several steps:

1. Replacing the PNP switch: If the Park/Neutral Position (PNP) switch is truly faulty, it should be replaced with a new original or quality replacement.
2. Repair or replace damaged wiring: If damage or breaks are found in the wiring connecting the PNP switch to the engine control module, the corresponding wires must be repaired or replaced.
3. Cleaning or replacing connectors: If corrosion or oxidation is found on the connector pins, they should be cleaned or replaced.
4. Diagnostics and replacement of the engine control module: If all the previous steps do not resolve the problem, the problem may be with the engine control module (PCM). In this case, it is necessary to perform additional diagnostics and, if necessary, replace the PCM.
5. Checking and servicing the transmission system: After fixing the PNP switch problem, you should also check the condition and operation of other components of the transmission system to rule out possible problems.

It is recommended that diagnosis and repair be carried out by a qualified auto mechanic or an authorized service center to ensure that the problem is corrected correctly and the vehicle is restored to normal operation.

What is P0851 Engine Code [Quick Guide]

P0851 – Brand-specific information

Deciphering the P0851 trouble code for some specific car brands:

1. Chevrolet:
   • P0851 – Park/Neutral Position Switch Input Circuit Low.
2. Ford:
   • P0851 – Park/Neutral Position Switch Input Circuit Low.
3. Toyota:
   • P0851 – Park/Neutral Position Switch Input Circuit Low.
4. Honda (Honda):
   • P0851 – Park/Neutral Position Switch Input Circuit Low.
5. Nissan (Nissan):
   • P0851 – Park/Neutral Position Switch Input Circuit Low.
6. BMW:
   • P0851 – Park/Neutral Position Switch Input Circuit Low.
7. Mercedes-Benz (Mercedes-Benz):
   • P0851 – Park/Neutral Position Switch Input Circuit Low.
8. Volkswagen:
   • P0851 – Park/Neutral Position Switch Input Circuit Low.

These transcripts describe that the cause of the P0851 trouble code is the park/neutral position switch input circuit being low for the specified vehicle brands.