Diagnostic Instructions

DTC Descriptors

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

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

Diagnostic Fault Information

Circuit	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
Park/Neutral Signal	P0851	P0852	P0852	--

Typical Scan Tool Data

PNP Switch - ECM

Circuit	Short to Ground	Open/High Resistance	Short to Voltage
Operating Conditions: Engine running, normal operating temperature Parameter Normal Range: In-Gear, Park/Neutral
Park/Neutral Signal	Park/Neutral	In-Gear	In-Gear

Circuit/System Description

The transmission manual shift shaft switch assembly, also known as the internal mode switch (IMS), is a sliding contact switch attached to the manual shift shaft inside the transmission case. The Park/Neutral position (PNP) switch is integrated into the IMS and connects to the transmission control module (TCM) lead-frame through a short wire harness. The 5 inputs to the TCM from the transmission manual shift shaft switch assembly indicate the transmission gear selector lever position. This information is used for engine start enable, engine controls, as well as determining the transmission shift patterns. The state of each input is available for display on the scan tool. The five input parameters represented are signal A, signal B, signal C, signal P (parity) and signal N (Park/Neutral start signal). Park/Neutral signal is sent from the Park/Neutral switch directly to the engine control module (ECM) and is used for engine start enable. The Park/Neutral signal circuit is not connected to the TCM internally, it uses the TCM as a pass-through connector only. The ECM Park/Neutral diagnostic monitors the Park/Neutral signal circuit to detect low voltage when the ECM expects to see high voltage and high voltage when the ECM expects to see low voltage.

Conditions for Running the DTC

P0851

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 and the following conditions are present:

Action Taken When the DTC Sets

Conditions for Clearing the DIC/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 between the IMS and the P/N Switch, 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

Connector End View Reference

DTC Type Reference

Electrical Information Reference

Scan Tool Reference

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. It is especially important to closely observe the Park and Neutral to Drive transitional positions while moving the shift lever and observing for possible misalignment or slack in the shifter mechanism.

Circuit/System Testing

1. Ignition OFF, transmission in Park or Neutral, disconnect the transmission 16-way connector.
2. Test for less than 1 ohm of resistance between the Park/Neutral signal circuit terminal 3 on the transmission side and the ground circuit terminal 5.

⇒	If greater than the specified range, test the signal circuit inside the transmission for an open/high resistance. If the circuit tests normal, test or replace the IMS.

3. Ignition ON, verify the scan tool parameter Park/Neutral displays In-gear.

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

4. Install a fused jumper between the signal circuit and ground. Verify the scan tool parameter Park/Neutral displays Park/Neutral.

⇒	If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.

Component Testing

1. Ignition OFF, transmission in Park or Neutral, disconnect the transmission 16-way connector and the IMS from the control solenoid (w/body and TCM) valve assembly.
2. Test for infinite resistance between the control solenoid (w/body and TCM) valve assembly 16-way connector pin 3 and the IMS connector pin on the control solenoid (w/body and TCM) valve assembly side with the switch in the Open position.

⇒	If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the control solenoid (w/body and TCM) valve assembly.

3. Test for infinite resistance between the IMS connector pin F and the IMS case ground with the switch in the Open position.

⇒	If not the specified value, replace the IMS.

4. Perform the Control Solenoid Valve and Transmission Control Module Assembly Inspection .

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.