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 2 5-Volt Reference 1 Circuit

P0641, P0533, P0107, P0106, P0452

P2127

P0641, P0533, P0108, P0106

--

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

P0651

P2122

P0651, P2135, P0335

--

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

P0641, P0532, P0452, P0107, P0106

P0532

P0641

--

Engine Oil Pressure (EOP) Switch (if equipped) 5-Volt Reference 1 Circuit

P0107, P0641, P0522, P0452, P0532

P0522

P0641

--

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

P0641, P0532, P0452, P0107, P0106

P0452

P0641

--

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

P0641, P0532, P0453, P0107, P0106

P0107, P0106

P0641

--

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

P0651, P0335

P0222, P0122

P0651

--

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

P0651

P0335

P0651

--

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

P0641, P0532, P0452, P0107, P0106

P0340, P0341

P0641, P0533, P0108, P0106

--

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

P0641, P0532, P0452, P0107, P0106

P0365

P0641, P0533, P0108, P0106

--

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

P0641, P0532, P0452, P0107, P0106

P0345

P0641, P0533, P0108, P0106

--

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

P0641, P0532, P0452, P0107, P0106

P0390

P0641, P0533, P0108, P0106

--

Circuit/System Description

The control module 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 control module 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 accelerator pedal position (APP) sensor 2
    • The air conditioning (A/C) refrigerant pressure sensor
    • The engine oil pressure (EOP) sensor
    • The camshaft position (CMP) sensor bank 1 intake
    • The CMP sensor bank 1 exhaust
    • The CMP sensor bank 2 intake
    • The CMP sensor bank 2 exhaust

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

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

Conditions for Running the DTC

    • DTCs P0601, P0602, P0603, P0604, P0605, P0606, P0607, P060D, P062F and P2610 are not set.
    • The ignition is ON.
    • The ignition voltage is more than 9.5 volts.
    • DTCs P0641 and P0651 run continuously when the above conditions are met.

Conditions for Setting the DTC

The engine control module (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

DTC P0641 and P0651 are Type A DTCs.

Conditions for Clearing the DTC

DTC P0641 and P0651 are Type A DTCs.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Engine Control Module Connector End Views
    •  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 Data Definitions

Circuit/System Verification

P0641

Observe the MAP sensor voltage parameter with a scan tool with the ignition ON and the engine OFF. The scan tool should displays 3.6-4.8 volts for a normally operating system.

P0651

With the ignition ON and the engine OFF, observe the APP sensor 1 voltage parameter with a scan tool. The scan tool should display 0.3-0.9 volt for a normally operating system.

Circuit/System Testing

Important: 

   • Additional DTCs will set when disconnecting the components.
   • A short to voltage on the signal circuit of certain components may cause this DTC to set.

  1. Ignition OFF, disconnect the harness connector of all appropriate sensors for the applicable DTC.
  2. Ignition ON, measure for 4.8-5.2 volts between one of the 5-volt reference circuits and ground.
  3. 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, or the signal circuit, for each component associated with the appropriate 5-volt reference circuit. If all circuits test normal, replace the control module.
  4. 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.
  5. If the voltage display changes more than the specified value when a component is connected, replace the component.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Fuel Tank Pressure Sensor Replacement
    •  Accelerator Pedal Position Sensor Replacement
    •  Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement
    •  Camshaft Position Sensor Replacement - Bank 1 (Right Side) Intake
    •  Camshaft Position Sensor Replacement - Bank 1 (Right Side) Exhaust
    •  Camshaft Position Sensor Replacement - Bank 2 (Left Side) Intake
    •  Camshaft Position Sensor Replacement - Bank 2 (Left Side) Exhaust
    •  Crankshaft Position Sensor Replacement
    •  Control Module References
    •  Throttle Body Assembly Replacement
    •  Manifold Absolute Pressure Sensor Replacement