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 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

--

Crankshaft Position Sensor 5-Volt Reference 2 Circuit

P0651

P0335

P0651

--

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

P0641

P0452

P0641

--

Intake Camshaft Position Sensor 5-Volt Reference 1 Circuit

P0641

P0340

P0641

--

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

P0641

P0107

P0641

--

Secondary Air Injection (AIR) Solenoid 5-Volt Reference 2 Circuit

P0651, P2432

P2431

P0651, P2431

--

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

P0651

P0122

P0651

--

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

P0651

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 intake 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
    • The secondary air injection (AIR) solenoid

Conditions for Running the DTC

    • DTCs P0601, P0602, P0603, P0604, P0605, P0606, P0607, and P2610 are not set.
    • 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.

Diagnostic Aids

It may be possible to locate the fault by disconnecting one component at a time from the 5-volt reference circuit while viewing the 5-Volt Reference circuit parameter on the scan tool. The scan tool parameter would change from Fault to OK when the source of the fault is disconnected.

If all 5-volt reference components have been disconnected and a Fault is still indicated, the fault may exist in the wiring harness.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Engine Controls Connector End Views
    •  Engine Control Module 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
    •  Scan Tool Data Definitions

Circuit/System Verification

With the ignition ON and the engine OFF, observe the 5-volt Reference 1 and 2 parameters . The scan tool should display 4.3-4.7 volts for a normally operating system.

Circuit/System Testing

Important: Additional DTCs will set when disconnecting the components.

  1. Ignition OFF, disconnect the harness connector of all appropriate sensors for the applicable DTC.
  2. Important: A short to voltage on the signal circuit of certain components may cause this DTC to set.

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

Repair Instructions

    •  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
    •  Camshaft Position Sensor Replacement - Intake
    •  Crankshaft Position Sensor Replacement
    •  Secondary Air Injection Check Valve Replacement

Repair Verification

With the ignition ON and the engine OFF, observe the 5-Volt Reference 1 and 2 parameters. The scan tool should display 4.3-4.7 volts for a normally operating system.