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 provide an overview of each diagnostic category. |
DTC Descriptors
DTC P0851: Park/Neutral Position (PNP) Switch Circuit Low VoltageDTC 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 | P0852 | P0851 | P0851 | -- |
Typical Scan Tool Data
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 engine control module (ECM)/powertrain control module (PCM) provides 12 volts to the park/neutral position (PNP) switch. The switch consists of 5 separate circuits, 4 of which indicate gear selector position to the transmission control module (TCM), and one that indicates park/neutral position to the ECM/PCM. When the transmission is in Park or Neutral, the switch closes and pulls the ECM/PCM voltage low, 0 volts. When the transmission is not in Park or Neutral, the ECM/PCM voltage is high, 12 volts. The ECM/PCM uses the switch in order to enable starter operation.
Conditions for Running the DTC
DTC P0851
| • | Ignition voltage is between 11-18 volts. |
| • | The ignition is ON. |
| • | DTC P0852 is not active. |
DTC P0852
| • | Ignition voltage is between 11-18 volts. |
| • | Ignition is ON. |
| • | DTC P0851 is not active. |
| • | No GMLAN error messages. |
| • | Engine speed is more than 650 RPM. |
| • | Actual engine torque is more than 120 N·m (89 lb ft). |
| • | Accelerator pedal position is more than 6 percent. |
| • | Vehicle speed is more than 24 km/h (15 mph). |
Conditions for Setting the DTC
DTC P0851
The ECM/PCM detects the Park/Neutral switch signal equals 12 volts (high) when the IMS reports a Park/Neutral range for 5 seconds.
DTC P0852
The ECM/PCM detects the Park/Neutral switch signal equals 0 volts (low) when the IMS reports a Drive range for 5 seconds and the following conditions are present:
| • | TP is equal to or greater than 6 percent. |
| • | Engine torque is equal to or greater than 120 N·m (89 lb ft). |
| • | Vehicle speed is equal or greater than 24 km/h (15 mph). |
Action Taken When the DTC Sets
| • | DTCs P0851 and P0852 are Type C DTCs. |
| • | The ECM/PCM uses IMS range for engine start-up. |
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
Automatic Transmission Controls Schematics
Connector End View Reference
| • | Engine Control Module Connector End Views for 6.6L (LLY/LB7) |
| • | Powertrain Control Module Connector End Views for 8.1L (L18) |
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
- Ignition OFF, transmission in Park or Neutral, disconnect the transmission 20-way connector.
- Test for less than 1 ohm of resistance between the Park/Neutral signal circuit terminal R on the transmission side and ground.
- Ignition On, verify the Park/Neutral parameter displays In-gear.
- Install a fused jumper between the signal circuit and ground. Verify the Park/Neutral parameter displays Park/Neutral.
| ⇒ | If greater than the specified range, test the signal circuit between the connector and the IMS for an open/high resistance. If the circuit tests normal, replace the IMS. |
| ⇒ | If not the specified value, test the signal circuit for a short to ground. If the circuit tests normal, replace the ECM/PCM. |
| ⇒ | 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/PCM. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
- ECM/PCM replacement. Refer to Control Module References for replacement, setup, and programming.
- IMS replacement. Refer to Manual Shift Shaft, Detent Lever, and Position Switch Assembly Replacement .
