GM Service Manual Online
For 1990-2009 cars only

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 P0850: Park/Neutral Position (PNP) Switch Circuit

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

Starter Inhibit (PNP) Circuit

P0850, P0851

P0850, P0851

P0850, P0852

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Circuit/System Description

The transmission range (TR) switch is an integral part of the transmission control module (TCM). As the gear range selector lever is moved, the state of the Starter Inhibit Circuit may change, causing the circuit to open or close. An open circuit indicates a drive range and a low voltage signal, 0 volts, at the engine control module (ECM). A closed circuit indicates PARK or NEUTRAL and a high voltage, 12 volts, signal at the ECM.

The ECM uses the Starter Inhibit circuit status from the TCM in order to enable starter operation.

Conditions for Running the DTC

    • Start Lock Driving voltage is within 6.0-16.0 volts.
    • Ignition voltage is greater than 6.5 volts.

Conditions for Setting the DTC

DTC P0850 and P0851

The ECM detects "ON" signal from the Starter Inhibit circuit while the TCM is commanding the Starter Inhibit circuit "OFF" for 0.5 seconds.

DTC P0852

The ECM detects "OFF" signal from the Starter Inhibit Circuit while the TCM is commanding the Starter Inhibit Circuit "ON" for 0.5 second.

Action Taken When the DTC Sets

DTC P0850, P0851, and P0852 are Type C DTCs.

Conditions for Clearing the DTC

DTC P0850, P0851, and P0852 are Type C DTCs.

Reference Information

Schematic Reference

Automatic Transmission Controls Schematics

Connector End View Reference

    •  Automatic Transmission Internal Connector End Views
    •  Automatic Transmission Related Connector End Views

Description and Operation

Electronic Component Description

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

    •  Diagnostic Trouble Code (DTC) Type Definitions
    •  Diagnostic Trouble Code (DTC) List/Type

Scan Tool Reference

    •  Scan Tool Output Controls
    •  Scan Tool Data List
    •  Scan Tool Data Definitions

Circuit/System Verification

  1. If there are any other transmission DTCs set, diagnose those DTCs first. Refer to Diagnostic Trouble Code (DTC) List - Vehicle.
  2. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

Circuit/System Testing

  1. Engine ON, observe the TR Sw. parameter while selecting each transmission range: P, R, N, D5, D4, D3, D2, and D1. Each selected transmission range should correspond to the correct scan tool PNP Switch parameter displayed.
  2. If not equal to the specified range, test the Starter Inhibit Circuit for a short to ground, an open/high resistance, or a short to voltage. If the circuit test normal, replace the TCM.
  3. Observe the PNP Switch parameter (Engine Electrical/Theft Data) while selecting each transmission range: P, R, N, D5, D4, D3, D2, and D1. Each selected transmission range should correspond to the correct scan tool PNP Switch parameter displayed (Park/Neutral/In Gear).
  4. If not equal to the specified range, test the Starter Inhibit Circuit for a short to ground, an open/high resistance, or a short to voltage. If the circuit tests normal, replace the ECM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Perform the Control Module References for ECM or TCM replacement, setup, or programming.