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

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

P0107, P0452, P0532, P0641, P1183, P2228

P2127

P0533, P0641, P2229, P2261

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

P0651

P2122

P0651

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

P0069, P0107, P0340, P0345, P0365, P0390, P0452, P0522, P0532, P0641¹, P1183, P2228

P0532

P0533, P0641¹, P2229, P2261

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

P0069, P0107, P0340, P0345, P0365, P0390, P0452, P0522, P0532, P0641, P1183, P2228

P0452

P0533, P0641¹, P2229, P2261

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

P0107, P0452, P0522, P0532, P0641, P1183, P2228

P0107

P0533, P0641¹, P2229, P2261

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

P0651

P0122, P0222

P0651

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

P0069, P0107, P0340, P0345, P0365, P0390, P0452, P0522, P0532, P0641¹, P1183, P2228

P0522

P0533, P0641¹, P2229, P2261

Supercharger Intake Air Pressure (SIAP) 5-Volt Reference 1 Circuit

P0069, P0107, P0340, P0345, P0365, P0390, P0452, P0522, P0532, P0641, P1183, P2228

P1183

P0533, P0641¹, P2229, P2261

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

P0651

P0335

P0651

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

P0069, P0107, P0340, P0345, P0365, P0390, P0452, P0522, P0532, P0641, P1183, P2228

P0365

P0533, P0641¹, P2229, P2261

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

P0069, P0107, P0340, P0345, P0365, P0390, P0452, P0522, P0532, P0641, P1183, P2228

P0340

P0533, P0641¹, P2229, P2261

Camshaft Position (CMP) Sensor Bank 2 Exhaust 5-Volt Reference 1 Circuit

P0069, P0107, P0340, P0345, P0365, P0390, P0452, P0522, P0532, P0641, P1183, P2228

P0390

P0533, P0641¹, P2229, P2261

Camshaft Position (CMP) Sensor Bank 2 Intake 5-Volt Reference 1 Circuit

P0069, P0107, P0340, P0345, P0365, P0390, P0452, P0522, P0532, P0641, P1183, P2228

P0345

P0533, P0641¹, P2261, P2229

¹Engine crank but does not start

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 air conditioning (A/C) refrigerant pressure sensor
    • The manifold absolute pressure (MAP) sensor
    • The fuel tank pressure (FTP) sensor
    • The accelerator pedal position (APP) sensor 2
    • The camshaft position (CMP) sensors
    • The Engine Oil Pressure (EOP) sensor
    • The Supercharger Intake Air Pressure (SIAP) 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

    • DTCs P0601, P0602, P0603, P0604, P0606, P0607, P060D, P062F, P1680 and P2610 are not set.
    • The ignition is ON.
    • 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

    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

Conditions for Clearing the MIL/DTC

    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

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

Scan Tool Reference

Control Module References for scan tool information

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 tot he 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
    •  Fuel Tank Pressure Sensor Replacement
    •  Manifold Absolute Pressure Sensor Replacement
    •  Throttle Body Assembly Replacement
    •  Control Module References for ECM replacement, setup, and programming