GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    •  Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provide an overview of each diagnostic category.

DTC Descriptors

DTC P0641: 5-Volt Reference 1 Circuit

DTC P0651: 5-Volt Reference 2 Circuit

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Accelerator Pedal Position (APP) Sensor 1 5-Volt Reference 2 Circuit

P0651

P2122

P0651

--

Accelerator Pedal Position (APP) Sensor 2 5-Volt Reference 1 Circuit

P0641

P2127

P0641

--

Air Conditioning (A/C) Refrigerant Pressure Sensor 5-Volt Reference 1 Circuit

P0641

P0532

P0641

--

Brake Booster Vacuum Sensor 5-Volt Reference 1 Circuit

P0641

P0557

P0641

--

Camshaft Position (CMP) Sensor 5-Volt Reference 1 Circuit

P0641

P0340

P0641

--

Engine Oil Pressure (EOP) Sensor 5-Volt Reference 1 Circuit

P0641

P0522

P0641

--

Fuel Tank Pressure (FTP) Sensor 5-Volt Reference 1 Circuit

P0641

P0452

P0641

--

Manifold Absolute Pressure (MAP) Sensor 5-Volt Reference 1 Circuit

P0641

P0107

P0641

--

The Crankshaft Position (CKP) Sensor 5-Volt Reference 2 Circuit

P0651

P0335

P0651

--

Throttle Position (TP) Sensor 1 and 2 5-Volt Reference 2 Circuit

P0651

P0122, P0222

P0651

--

Circuit/System Description

The engine control module (ECM) has 2 internal 5-volt reference busses called 5-volt reference 1 and 5-volt reference 2. Each reference bus provides 5-volt reference circuits for more than one sensor. Therefore, a fault condition on one 5-volt reference circuit will affect the other 5-volt reference circuits connected to that reference bus. The ECM monitors the voltage on the 5-volt reference busses.

The 5-volt reference 1 bus provides 5 volts to the following sensors:

    • The manifold absolute pressure (MAP) sensor
    • The fuel tank pressure (FTP) sensor
    • The air conditioning (A/C) refrigerant pressure sensor
    • The accelerator pedal position (APP) sensor 2
    • The engine oil pressure (EOP) sensor
    • The camshaft position (CMP) sensor

The 5-volt reference 2 bus provides 5 volts to the following sensors:

    • The APP sensor 1
    • The throttle position (TP) sensor 1 and 2
    • The crankshaft position (CKP) sensor

Conditions for Running the DTC

    • The ignition is in Unlock, Accessory, Run, or Crank.
    • The ignition voltage is more than 5.23 volts.
    • DTCs P0641 and P0651 run continuously when the above conditions are met.

Conditions for Setting the DTC

The ECM detects a voltage out of tolerance condition on the 5-volt reference 1 or 2 bus for more than 0.5 second.

Action Taken When the DTC Sets

DTCs P0641 and P0651 are Type A DTCs.

Conditions for Clearing the MIL/DTC

DTCs P0641 and P0651 are Type A DTCs.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Engine Controls Connector End Views

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

    •  Scan Tool Data List
    •  Scan Tool Output Controls

Circuit/System Verification

Ignition ON, observe the scan tool 5-volt reference 1 and 2 parameters. The reading should display Circuit OK for a normally operating system.

Circuit/System Testing

Important: Additional DTCs may set when disconnecting components.

  1. Ignition OFF, disconnect the harness connector of all the sensors connected to the 5-volt reference bus for the applicable DTC. Refer to Circuit/System Description.
  2. Important: A short to voltage on the signal circuit of certain components may cause this DTC to set.

  3. Ignition ON, test for 4.8-5.2 volts between one of the 5-volt reference circuits and ground.
  4. If less than the specified range, test all of the 5-volt reference circuits for a short to ground. If all circuits test normal, replace the ECM.
    If greater than the specified range, test for a short to voltage on all of the 5-volt reference circuits, or the signal circuit for each component associated with the appropriate 5-volt reference bus for a short to voltage. If all circuits test normal, replace the ECM.
  5. Connect each component associated with the appropriate 5-volt reference bus, one at a time, while monitoring the voltage. The voltage should not change more than 1 volt.
  6. If the voltage display changes more than the specified range when a component is connected, replace the component.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Accelerator Pedal Position Sensor Replacement
    •  Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement
    •  Control Module References for ECM replacement, setup, and programming
    •  Fuel Tank Pressure Sensor Replacement
    •  Manifold Absolute Pressure Sensor Replacement
    •  Throttle Body Assembly Replacement
    •  Engine Oil Pressure Sensor and/or Switch Replacement
    •  Camshaft Position Sensor Replacement
    •  Crankshaft Position Sensor Replacement