P0478

DTC P0478

CIRCUIT DESCRIPTION
The engine control module (ECM) controls the exhaust brake control relay which supplies power to the intake throttle solenoid valve and exhaust brake magnetic valve based on vehicle running condition. The ECM monitors the exhaust brake control relay operating status through the exhaust brake relay feedback circuit and the intake throttle solenoid feedback circuit. If the ECM detects a high voltage condition on exhaust brake control relay control circuit or either of the feedback circuits when the exhaust brake is commanded ON, this DTC will set.

Condition for Running the DTC
- DTCs P0477, P0642, P0643, U0073 and U0101 are not set.
- The battery voltage is more than 10-16 volts.
- The ignition switch is ON time is longer than 5 seconds.
- The engine is running.
- The exhaust brake cut is not controlled.

Condition for Setting the DTC
- The ECM detects a high voltage condition on the exhaust brake control relay control circuit for longer than 1 second.

OR
- The ECM detects a high voltage condition on the exhaust brake control relay feedback circuit or intake throttle solenoid valve feedback circuit for longer than 1.5 seconds when the exhaust brake control relay is commanded OFF.

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 injection quantity within 50%.
- The ECM limits fuel rail pressure within 11600 psi (80MPa).
- The ECM closes the EGR valve and holds to close position.
- The ECM limits accelerator pedal control range within 40%.
- 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.
- 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:
- Faulty exhaust brake control relay
- Misrouted harness
- Rubbed through wire insulation
- Broken wire inside the insulation

Check for the following conditions:
- Damaged harness: Inspect the wiring harness for damage. If the harness appears to be OK, observe the "Exhaust Brake Relay" or "Intake Throttle Solenoid" display on the scan tool with engine running while moving connectors and wiring harness. A change in the display will indicate the location of the fault.

NOTE: If the exhaust brake control relay control circuit is open circuit, DTC P0477 will also set.

DTC P0478
Schematic Reference: Engine Controls Schematics
Connector End View Reference: Engine Controls Connector End Views or Engine Control Module (ECM) Connector End Views

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