P0643

DTC P0643

CIRCUIT DESCRIPTION
The engine control module (ECM) provides 5 volts reference voltage through the reference circuit 1 to the following sensors:
- Accelerator pedal position (APP) sensor 1
- PTO throttle sensor
- Idle up sensor
- Boost pressure sensor
- Fuel rail pressure sensor
- EGR valve position sensor

The 5 volts reference circuits are independent of each other outside the ECM, but are bussed together inside the ECM. Therefore, a short circuit condition on one sensor 5 volts reference circuit may affect the entire sensor 5 volt reference circuit 1. The ECM monitors the voltage on the 5 volts reference circuit 1. If the ECM detects the voltage is excessively high, this DTC will set.

Condition for Running the DTC
- DTC P0642 is not set.
- The battery voltage is between 10-16 volts.
- The ignition switch is ON.

Condition for Setting the DTC
- The ECM detects that the 5 volts reference circuit 1 voltage is more than 5.5 volts.

Action Taken When the DTC Sets
- The ECM illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
- The ECM records the operating conditions at the time the diagnostic fails. The ECM stores this information in the Freeze Frame/Failure Records.
- The ECM limits fuel rail pressure within 11600 psi (80MPa).
- The ECM limits accelerator control range within 40%.
- The ECM closes the EGR valve and holds to close position.
- The ECM inhibits the cruise control.
- The ECM inhibits the PTO control.

Condition for Clearing the MIL/DTC
- The ECM turns OFF the MIL after 3 consecutive driving cycles when the diagnostic runs and does not fail. (12,000 lbs GVW)
- The ECM turns OFF the MIL when the diagnostic runs and does not fail. (Except 12,000 lbs GVW)
- 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 after the ECM turns OFF the MIL.
- Clear the MIL and the DTC with a scan tool.

Diagnostic Aids
If an intermittent condition is suspected, the following may cause an intermittent:
- Misrouted harness
- Rubbed through wire insulation

Check for the following conditions:
- Damaged harness: Inspect the wiring harness for damage. If the harness appears to be OK, observe the "5 Volt Reference 1" display on the scan tool with ignition ON, engine OFF while moving connectors and the wiring harness related to the sensor. A change in the display will indicate the location of the fault.

DTC P0643
Schematic Reference: Engine Controls Schematics
Connector End View Reference: Engine Controls Connector End Views or Engine Control Module (ECM) Connector End Views

Step 3 - 7:

Step 8 - 13:

Step 14 - 17: