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DTC P0401: EGR System Failure: Testing

  1. Turn the ignition on. With the DRBIII®, read DTCs. Is the Good Trip counter displayed and equal to zero? If yes, go to next step. If no, go to step  12.
  2. Start the engine. Allow the engine to reach operating temperature. Does the engine run rough or stall at idle? If yes, go to next step. If no, go to step  6.
  3. Turn the ignition off. Disconnect the vacuum hose from the EGR Valve. Start the engine and allow it to idle. Does the engine run rough or stall at idle? If yes, remove the EGR Valve and inspect the valve for an obstruction that would keep the valve open. If okay, replace the EGR Valve. If no, go to next step.
  4. Turn the ignition off. Disconnect the PCM C1 harness connector. Disconnect the EGR Solenoid harness connector. Measure the resistance of the EGR Solenoid Control circuit in the EGR Solenoid harness connector to ground. Is the resistance below 5.0 ohms? If yes, repair the EGR Solenoid Control circuit for a short to ground. If no, go to next step.
  5. Turn the ignition off. Reconnect PCM connector. Using a 12-volt test light connected to 12-volts, probe the EGR Solenoid Control circuit at the EGR Solenoid harness connector. Turn the ignition on. With the DRBIII®, actuate the EGR Solenoid. Observe the test light. Does the test light flash on and off? If yes, replace the EGR Solenoid. If no, replace the PCM. Program the new PCM. See PROGRAMMING .
  6. Turn the ignition off. Disconnect the EGR Solenoid vacuum supply at the EGR Solenoid. Connect a vacuum gauge to the EGR Solenoid vacuum supply. Start the engine. Observe the vacuum gauge. Does the vacuum gauge read above 10 in. Hg? If yes, go to next step. If no, repair the EGR Solenoid vacuum supply.
  7. Disconnect the vacuum hose from the EGR Valve. Inspect the vacuum hose for any obstructions. Connect a Vacuum Pump to the EGR Valve. Using the Vacuum Pump, apply vacuum to the EGR Valve. Does the engine run rough or stall at idle? If yes, go to next step. If no, remove the EGR Valve and inspect the valve for an obstruction that would keep the valve from opening. If okay, replace the EGR Valve.
  8. Turn the ignition off. Disconnect the EGR Solenoid harness connector. Turn the ignition on. Using a 12-volt test light connected to ground, probe the Fused Ignition Switch Output circuit in the EGR Solenoid harness connector. Does the 12-volt test light illuminate? If yes, go to next step. If no, repair the Fused Ignition Switch Output circuit for an open.
  9. Measure the voltage of the EGR Solenoid Control circuit in the EGR Solenoid harness connector. Is the voltage above 1.0 volt? If yes, repair the EGR Solenoid Control circuit for a short to voltage. If no, go to next step.
  10. Turn the ignition off. Disconnect the PCM C1 harness connector. Measure the resistance of the EGR Solenoid Control circuit between the PCM C1 harness connector and the EGR Solenoid harness connector. Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the EGR solenoid control circuit for an open.
  11. Turn the ignition off. Reconnect PCM connector. Using a 12-volt test light connected to 12-volts, probe the EGR Solenoid Control circuit in the EGR Solenoid harness connector. Turn the ignition on. With the DRBIII®, actuate the EGR Solenoid. Observe the test light. Does the test light flash on and off? If yes, inspect the EGR Solenoid Back Pressure hose and tube obstructions or damage. If okay, replace the EGR Solenoid. If no, replace the PCM. Program the new PCM. See PROGRAMMING .
    12. WARNING: When the engine is operating, do not stand in a direct line with the fan. Do not put your hands near the pulleys, belts or fan. Do not wear loose clothing.
  12. The conditions that set the DTC are not present at this time. The following may help in identifying the intermittent condition. With the engine running at normal operating temperature, monitor the DRBIII® parameters related to the DTC while wiggling the wiring harness. Look for parameter values to change and/or a DTC to set. Review the DRBIII® Freeze Frame information. If possible, try to duplicate the conditions under which the DTC was set. Refer to any Technical Service Bulletins (TSB) that may apply. Visually inspect the related wiring harness. Look for any chafed, pierced, pinched, or partially broken wires. Visually inspect the related wiring harness connectors. Look for broken, bent, pushed out, or corroded terminals. Were any of the above conditions present? If yes, repair as necessary. If no, test is complete.
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