DTC P0851 or P0852

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 Descriptors

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

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

Diagnostic Fault Information



Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Park/Neutral Signal





Scan Tool Parameter Data


PNP Switch


Short to Ground

Open/High Resistance

Short to Voltage

Operating Conditions: Engine running, normal operating temperature

Parameter Normal Range: 12 Volts = In-Gear, 0 Volts = Park/Neutral

Park/Neutral Signal




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; the circuit 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



Ignition voltage is between 9-18 volts.



No TCM DTCs P0601, P0602, P0603, or P0604.



No IMS DTCs P1825 or P1915.



No Communication DTCs U0001, U0100, U0101, U2105, or U2106.



No TP DTCs P0121, P0122, P0123.



No OSS DTCs P0722 or P0723.



No MAF DTCs P0101, P0102, or P0103.



No MAP DTCs P0106, P0107, or P0108.



Engine Speed is greater than 400 RPM.

Conditions for Setting the DTC




The ECM detects the Park/Neutral switch signal equals 0 volts (Low) when the IMS reports a Drive range for 0.2 seconds.



TP is equal to or greater than 10 percent.



Engine torque is equal to or greater than 75 Nm (55 lb ft).



Vehicle speed is equal or greater than 10 km/h (6 mph).


The ECM detects the Park/Neutral switch signal equals 12 volts (High) when the IMS reports a Park/Neutral range for 0.2 seconds.

Action Taken When the DTC Sets



DTC P0851 and P0852 are Type C DTCs.



The ECM uses IMS Range for engine start-up.

Conditions for Clearing the DTC


DTC P0851 and P0852 are Type C DTCs.

Reference Information


Schematic Reference


Automatic Transmission Controls Schematics



Engine Controls Schematics

Connector End View Reference


Automatic Transmission Inline 16-Way Connector End View



Automatic Transmission Internal Connector End Views



Automatic Transmission Related Connector End Views

DTC Type Reference


Diagnostic Trouble Code (DTC) Type Definitions



Diagnostic Trouble Code (DTC) List/Type

Electrical Information Reference


Circuit Testing



Connector Repairs



Testing for Intermittent Conditions and Poor Connections



Wiring Repairs

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 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. If any concerns are found proceed to Testing.

Circuit/System Testing


Disconnect the transmission 16-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 16-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 from the IMS connector on the TCM side to the 16-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, or programming.

Component Testing


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

Repair Instructions


Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

  1. Engine Control Module (ECM) Assembly replacement - refer to Control Module References for replacement, setup, and programming.
  2. Internal mode switch (IMS) Replacement - refer to Manual Shift Shaft Position Switch Replacement .