The park/neutral position (PNP) switch is externally mounted on the transmission manual shaft. The switch consists of five separate circuits, four of which indicate gear selector position to the transmission control module (TCM), and one that indicates park/neutral position to the ECM. As the gear range selector lever is moved, the state of the P/N switch may change, causing the park/neutral signal circuit to open or close. An open circuit or switch indicates a drive range and a low voltage, 0 volts, signal at the engine control module (ECM). A closed circuit or switch indicates PARK or NEUTRAL and a high voltage, 12 volts, signal at the ECM. The ECM uses the PNP Switch status in order to enable starter operation.

P0851

The ECM detects the Park/Neutral switch signal equals 12 volts, high, when the PNP switch reports a Park/Neutral range for 3 seconds.

P0852

The ECM detects the Park/Neutral switch signal equals 0 volts, low, when the PNP switch reports a Drive range for 3 seconds.

DTCs P0851 and P0852 are Type C DTCs.

It is possible for this DTC to set due to a stack-up or misalignment issue, where no electrical fault is present. Ensure the gear shift selector cable is properly adjusted and there are no service bulletins that address this concern before replacing parts.

Schematic Reference

Automatic Transmission Controls Schematics

Connector End View Reference

Description and Operation

Electronic Component Description for park/neutral position switch information

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. If there are any other transmission related DTCs set, diagnose those DTCs first. Refer to Diagnostic Trouble Code (DTC) List - Vehicle.
2. Ignition ON, observe the scan tool ECM PNP switch parameter while moving the gear shift lever from Park, to Reverse and through all the ranges. The parameter should display Park/Neutral when in Park or Neutral and In-Gear when in Reverse, Drive, or Drive Low.
3. Observe the scan tool ECM PNP switch parameter while slowly moving the gear shift lever from Park, to Reverse and through all the ranges. Verify the gear position matches the scan tool parameter.

⇒	If not the specified value, refer to Shift Control Cable Adjustment.

4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

1. Ignition OFF, disconnect the harness connector at the transmission range switch.
2. Using the terminal test kit, connect a test lamp between battery positive voltage and terminal 12 of the PNP switch. Shift the transmission into Park and Neutral the test lamp should illuminate.

⇒	If the test lamp does not illuminate when in Park or Neutral, replace the PNP switch.

3. Shift the transmission into Reverse and all other Drive ranges, the test lamp should not illuminate.

⇒	If the test lamp illuminates, replace the PNP switch.

4. Ignition ON, verify the scan tool ECM PNP switch parameter displays In-Gear.

⇒	If not the specified value, test the park/neutral signal circuit for a short to ground. If the circuit tests normal, replace the ECM.

5. Install a 3A fused jumper wire between the park/neutral signal circuit terminal 12 and ground. Verify the scan tool ECM PNP switch parameter displays Park/Neutral.

⇒	If not the specified value, test the park/neutral signal circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

Perform the Diagnostic Repair Verification after completing the repair.