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 3 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 for more than 3 seconds.

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

### Conditions for Setting the DTC

The 5-volt reference 1 voltage is above or below a predetermined threshold for more than 4 seconds.

### 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

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

• | The fuel tank pressure (FTP) sensor

• | The engine oil pressure (EOP) sensor

### 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

• | Scan Tool Output Controls

### Circuit/System Verification

1. Ignition ON, observe the DTC information with a scan tool. DTCs P0642 and P0643 should not set.
2. If the vehicle passes the Circuit/System Verification test, operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records List.

### Circuit/System Testing

Important: Additional DTCs will set when disconnecting the components.

1. Ignition OFF, disconnect the harness connector of all appropriate sensors for the applicable DTC. Refer to Diagnostic Aids.
2. Ignition ON, measure for 4.6-5.2 volts between one of the 5-volt reference circuits and ground.

⇒ | If the voltage is less than 4.6 volts, 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 5.2 volts, test for a short to voltage on all the 5-volt reference circuits for each component associated with the appropriate 5-volt reference circuit. If all circuits test normal, replace the control module.

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 1 circuit one at a time while monitoring the 5-Volt Reference 1 Circuit Status parameter with a scan tool. The parameter should continue to display OK.

⇒ | If the parameter displays Fault when a component is connected, test the signal circuit of that component for a short to voltage. If the circuit tests normal, replace the component.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Fuel Tank Pressure Sensor Replacement

• | Engine Oil Pressure Sensor and/or Switch Replacement

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