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 diagnostic category.

### DTC Descriptors

DTC P0642: 5-Volt Reference 1 Circuit Low Voltage
DTC P0643: 5-Volt Reference 1 Circuit High Voltage

### Circuit/System Description

The engine control module (ECM) has 2 5-volt reference circuits. The ECM provides the 5-volt reference to various sensors. Each reference circuit provides a 5-volt reference for more than one sensor. A short to ground or voltage on one of the 5-volt reference circuits will affect all components connected to that 5-volt reference circuit. The ECM monitors the voltage on the 5-volt reference circuits.

### Conditions for Running the DTC

• | The ignition is ON.

• | DTCs P0642 and P0643 run continuously once the above condition is met.

### Conditions for Setting the DTC

The control module detects a voltage above 5.2 volts, or below 4.8 volts, on either 5-volt reference circuit for greater than 1 second.

### Action Taken When the DTC Sets

DTCs P0642 and P0643 are Type A DTCs.

### Conditions for Clearing the DTC

DTCs P0642 and P0643 are Type A DTCs.

### Diagnostic Aids

Important: An internal ECM 5-volt regulator supplies 5 volts to the 5-volt reference circuits and the fuel level sensor signal circuit. When an external 5-volt reference 1 circuit is shorted, DTC P0462 or P0463 sets even though there is no condition with the fuel level sensor.

The 5-volt reference 1 circuit provides 5 volts to the following sensors:

• | The accelerator pedal position (APP) sensor 1

• | The APP sensor 2

• | The air conditioning (A/C) refrigerant pressure sensor

• | The throttle position (TP) sensor 1

• | The TP sensor 2

If a shared 5-volt reference circuit is shorted to ground or shorted to a voltage, other 5-volt reference circuits may be affected.

### 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. Ignition ON, observe the DTC information with a scan tool. DTCs P0652 and P0653 should not set.

⇒ | If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.

2. Observe the DTC information with a scan tool. DTCs P0642 and P0643 should not set.
3. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

### Circuit/System Testing

Important: Additional DTCs will set when disconnecting the components.

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

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

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

3. Connect each component associated with the 5-volt reference circuit one at a time while observing the voltage. The voltage should not change greater than 0.5 volt.

⇒ | If less than the specified range when a component is connected, replace the component.

⇒ | If greater than the specified range when a component is connected, test the signal circuit for a short to voltage. If all circuits test normal, replace the ECM.

### 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

• | Throttle Body Assembly Replacement

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