GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The engine control module (ECM) provides 5 volts to the following sensors:

    • The oil pressure sensor
    • The exhaust gas recirculation (EGR) valve
    • The accelerator pedal position (APP) 1 sensor
    • The rail pressure sensor

These 5-volt reference circuits are independent of each other outside the ECM, but are bussed together inside the ECM. Therefore, a circuit condition on one sensor 5-volt reference circuit may affect the other sensor 5-volt reference circuits. The ECM monitors the voltage on the 5-volt reference circuit. If the ECM detects a high voltage condition, DTC P0643 sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0643 5-Volt Reference 1 Circuit High Voltage

Conditions for Running the DTC

The ignition is ON.

Conditions for Setting the DTC

    • The ECM detects a voltage out of tolerance condition on the 5-volt reference A circuit.
    • The above condition is present for longer than 10 seconds.

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 Freeze Frame/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.
    • Clear the MIL and the DTC with a scan tool.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check - Engine Controls?

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Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Turn ON the ignition, with the engine OFF.
  2. Observe the 5-Volt Reference 1 parameter with a scan tool.

Is the 5-Volt Reference 1 parameter more than the specified value?

4.7 V

Go to Step 3

Go to Intermittent Conditions

3

Test each 5-volt reference 1 circuits for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 6

Go to Step 4

4

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 6

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5

  1. Clear the 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 the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 6

6

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK