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 is Open

• | DTC P0642 5-volt Reference 1 Circuit Low Voltage

• | DTC P0643 5-volt Reference 1 Circuit High Voltage

### Circuit Description

The control module has 3 internal 5-volt reference busses called 5-volt reference 1, 5-volt reference 2 and 5-volt reference 3. 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 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 air conditioning (A/C) refrigerant pressure sensor;

• | The mass air flow (MAF)/intake air temperature (IAT) sensor;

• | The fuel tank pressure (FTP) sensor;

• | The Accelerator Pedal Position (APP) sensor 2;

• | The engine oil pressure (EOP) sensor;

• | The barometric pressure (BARO) sensor;

### Conditions for Running the DTC

• | Turn the ignition switch ON;

• | The engine control module (ECM) power relay is turned ON;

• | P0641, P0642 or P0643 continuously run once the above condition is present for 3 seconds.

### Conditions for Setting the DTC

The control module detects for an open, a short to ground or a short to the voltage on the 5-volt reference circuit 1.

### Action Taken When the DTC Sets

• | The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.

• | The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.

### Conditions for Clearing the MIL/DTC

• | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.

• | A current DTC, last test failed, clears when the diagnostic runs and passes.

• | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.

### Diagnostic Aids

• | Use the J 35616-A/BT-8637 Connector Test Adapter Kit for any test that requires probing the ECM harness connector or a component harness connector.

• | The lower connector of the ECM is connector C1 and the upper connector of the ECM is connector C2. Refer to Front of Vehicle/Engine Compartment Component Views.

• | For an intermittent condition, refer to Intermittent Conditions.

Step | Action | Value | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics Connector End View Reference: Component Connector End Views
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 |

1. Observe the Freeze Frame/Failure Records for this DTC.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC. You may also operate he vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Does the DTC fail this ignition?

--

Go to Step 3

Go to Diagnostic Aids

3 |

• | Disconnect the Mass Air Flow (MAF)/intake Air Temperature (IAT) sensor.

• | Turn the ignition switch ON, with the engine OFF. Measure the voltage parameter between the MAF/ IAT sensor terminal 4 on the 5-volt reference circuit and the ground.

Is the voltage within the specified value?

4.8-5.2 V

Go to Step 10

Go to Step 4

4 | Is the voltage more than the specified value? | 5.2 V | Go to Step 5 | Go to Step 6
5 |

Note: If the 5-volt reference circuit is shorted to voltage, the A/C refrigerant pressure sensor may be damaged. If the fuel tank pressure (FTP) sensor signal circuit is short to voltage, the FTP sensor may be damaged.

1. Test for a short to voltage on the 5-volt reference circuits of all the sensors connected to the 5-volt reference voltage 1 bus.
2. Test the signal circuit of the FTP sensor for a short to voltage. Refer to Circuit Testing and Wiring Repairs.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 8

6 | Is the voltage less than the specified value? | 4.8 V | Go to Step 7 | --
7 | Test for a short to ground on the 5-volt reference circuits of all the sensors connected to the 5-volt reference voltage 1 bus. Refer to Circuit Testing and Wiring Repairs. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 8
8 |

1. Measure continuously the voltage parameter between the MAF/IAT sensor terminal 4 on the 5-volt reference circuit and the ground.
2. Observe the DMM while disconnecting all other sensors connected to the 5-volt reference 1 bus, one at a time.

When a sensor is disconnected, if the voltage returns to the specified value.

4.8-5.2 V

Go to Step 9

Go to Step 13

9 | Replace the sensor. Refer to appropriate sensor replacement. | -- | Go to Step 10 | --
10 |

1. Reconnect all sensors except the A/C refrigerant pressure sensor. If A/C refrigerant pressure sensor is still connected, disconnect it.
2. Measure the Voltage parameter between the 5-volt reference circuit of the A/C refrigerant pressure sensor and the ground with a DMM.

Is the voltage within the specified range?

4.8-5.2 V

Go to Step 14

Go to Step 11

11 | Test for a short to voltage of the MAF/IAT sensor signal circuit or malfunction of the MAF/IAT sensor. Refer to Circuit Testing and Wiring Repairs. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 12
12 | Replace the mass air flow (MAF)/intake air temperature (IAT) sensor. Refer to Mass Airflow Sensor with Intake Air Temperature Sensor Replacement. Did you complete the replacement? | -- | Go to Step 14 | --
13 | Replace the ECM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 14 | --
14 |

1. Clear any DTCs with a scan tool.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC. You may also operate he vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Does the DTC fail this ignition?

--

Go to Step 2

Go to Step 15

15 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle | System OK