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 provides an overview of each of the 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 | P2122 | P0651, P2123 | --
Accelerator Pedal Position (APP) Sensor 2 5-Volt Reference 1 Circuit | P0641, P2127 | P2127 | P0641, P2128 | --
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 | --
Exhaust Camshaft Position Sensor 5-Volt Reference 1 Circuit | P0641, P0365 | P0365 | P0641, P0365 | --
Fuel Tank Pressure (FTP) Sensor 5-Volt Reference 1 Circuit | P0641 | P0452 | P0641 | --
Intake Camshaft Position Sensor 5-Volt Reference 1 Circuit | P0641, P0340 | P0340 | P0641, P0340 | --
Manifold Absolute Pressure (MAP) Sensor 5-Volt Reference 1 Circuit | P0641, P0107 | P0107 | P0641, P0107 | --
Throttle Position (TP) Sensor 1 5-Volt Reference 2 Circuit | P0651, P0122 | P0122 | P0651, P0123 | --
Throttle Position (TP) Sensor 2 5-Volt Reference 2 Circuit | P0651, P0222 | P0222 | P0651, P0223 | --

### 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 buss provides 5-volt reference circuits for more than one sensor. A fault condition on one 5-volt reference circuit will affect the other 5-volt reference circuits connected to that reference buss. The ECM monitors the voltage on the 5-volt reference buss.

The 5-volt reference 1 buss 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 exhaust camshaft position (CMP) sensor

• | The intake CMP sensor

The 5-volt reference 2 buss 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, 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 range condition on the 5-volt reference 1 or 2 buss 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

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. With the ignition ON and the engine OFF, observe the 5-volt Reference 1 and 2 parameters. The scan tool should display 4.8-5.2 volts for a normally operating system.
2. Ignition ON, observe the DTC information with a scan tool. DTCs 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

1. Ignition OFF, disconnect the harness connector of all appropriate sensors for the applicable DTC.

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

2. Ignition ON, measure for 4.8-5.2 volts between one of the 5-volt reference circuits and ground.

⇒ | 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 ECM.

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

### Repair Instructions

• | Accelerator Pedal with Position Sensor Assembly Replacement

• | Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement

• | Camshaft Position Sensor Replacement - Exhaust

• | Camshaft Position Sensor Replacement - Intake

• | Crankshaft Position 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

### Repair Verification

With the ignition ON and the engine OFF, observe the 5-Volt Reference 1 and 2 parameters. The scan tool should display 4.8-5.2 volts for a normally operating system.