For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptor

DTC P0851: Park/Neutral Position (PNP) Switch Circuit Low Voltage
DTC P0852: Park/Neutral Position (PNP) Switch Circuit High Voltage

### Circuit/System Description

The engine control module (ECM) provides 12 volts to the park/neutral switch of the manual shift shaft position switch. The manual shift shaft position switch is a sliding contact assembly that attaches to the manual shift shaft inside the transmission. 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. When the transmission is in PARK or NEUTRAL, the switch closes and pulls the ECM voltage low, 0 volts. When the transmission is not in PARK or NEUTRAL, the ECM voltage is high, 12 volts. The ECM uses the switch in order to enable starter operation.

### Conditions for Running the DTC

• | Ignition voltage is between 8-18 volts.

• | No IMS DTCs P1825 or P1915.

• | The engine speed is 1,000 RPM or greater.

• | The engine torque is at least 75 N·m (55 lb ft).

• | The throttle position is 8 percent or greater.

• | Vehicle speed is 10 km/h (6 mph) or greater.

### Conditions for Setting the DTC

#### P0851

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

#### P0852

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

### Action Taken When the DTC Sets

• | DTCs P0851 and P0852 are Type C DTCs.

• | The ECM uses IMS range for engine start-up.

### Conditions for Clearing the DTC

DTCs P0851 and P0852 are Type C DTCs.

### Diagnostic Aids

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.

### Reference Information

#### Schematic Reference

Automatic Transmission Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Transmission General Information

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Transmission Control Module Scan Tool Information

#### Special Tools

J 45681 Jumper Harness

### Circuit/System Verification

If there are any other engine or transmission related DTCs set, diagnose those DTCs first. With a scan tool observe the Park/Neutral switch parameter in the engine controls data list and move the gear shift lever from Park, to Reverse and through all the ranges and observe the parameter as you move the shift lever. The parameter should display Park/Neutral when in Park or Neutral and In-Gear when in Reverse, Drive or Drive Low. Close observation of the Park and Neutral to Drive transitional positions while moving the shift lever and observing for possible misalignment or slack in the shifter mechanism.

⇒ | If any concerns are found proceed to Testing.

### Circuit/System Testing

Disconnect the transmission 20-way connector and observe the Park/Neutral scan tool parameter in the engine controls data list. The parameter should display In-Gear.

⇒ | If the Park/Neutral parameter does not display In-Gear test the Park/Neutral signal circuit between the engine side of the transmission 20-way connector and the ECM connector for a short to ground.

⇒ | If the Park/Neutral parameter displays In-Gear test the transmission IMS for a shorted P/N circuit to ground. Inspect the harness for damage or chaffing.

⇒ | If the IMS was not shorted to ground, test the IMS Park/Neutral signal circuit form the IMS connector on the TCM side of the 20-way connector for an open or short to ground.

⇒ | If no concerns are found, replace the ECM. Refer to Control Module References for replacement, setup, and programming.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Refer to Control Module References for replacement, setup, and programming.

• | Manual Shift Detent Lever with Shaft Position Switch Assembly Replacement

• | Torque Converter Clutch Pulse Width Modulation Solenoid, Torque Converter Clutch Solenoid, and Wiring Harness