For 1990-2009 cars

### 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 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 | --
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

• | DTCs P0601, P0602, P0603, P0604, P0605, P0606, P0607, P060D, P062F 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

• | 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 injector reaction (AIR) pressure sensor, if equipped

- | The engine oil pressure (EOP) switch

- | The air conditioning (A/C) refrigerant pressure 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

• | It may be possible to locate the fault by disconnecting one component at a time from the affected 5-volt reference circuit while viewing the 5-Volt Reference circuit status parameter on the scan tool. The scan tool parameter will 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

Component 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

Control Module References for scan tool information

### Circuit/System Verification

1. Observe the scan tool 5-volt reference 1 circuit status and the 5-volt reference 2 circuit status parameters. Verify the parameters display OK.
2. Ignition ON, observe the DTC information with a scan tool. DTC P0641 and P0651 should not set on this ignition cycle.
3. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

### Circuit/System Testing

Important: A short to voltage on the signal circuit of certain components may cause this DTC to set.

1. Ignition ON, disconnect the harness connector of all appropriate sensors for the applicable DTC. Refer to Diagnostic Aids for the appropriate sensors.
2. Ignition ON, measure for 4.8-5.2 volts between one of the 5-volt reference circuits and ground.

⇒ | If less than the specified range, test for a short to ground on all 5-volt reference circuits. If all circuits test normal, replace the ECM.

⇒ | If greater than the specified range, test for a short to voltage on all the 5-volt reference circuits. If all circuits test normal, replace the ECM.

3. Connect each component associated with the 5-volt reference circuit, one at a time, while monitoring the voltage. Verify the voltage remains between 4.8-5.2 volts.

⇒ | If not within the specified range when a component is connected, test the signal circuit for a short to voltage. If the circuit tests normal, replace the appropriate 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

• | Camshaft Position Sensor Replacement

• | Crankshaft Position Sensor Replacement

• | Engine Oil Pressure Sensor and/or Switch Replacement

• | Fuel Tank Pressure Sensor Replacement

• | Manifold Absolute Pressure Sensor Replacement

• | Throttle Body Assembly Replacement

• | Control Module References for ECM replacement, setup, and programming