Test Description

The numbers below refer to the step numbers in the diagnostic procedures.

1. 1 -  The Diagnostic System Check - Engine Controls prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
2. 3 -  This step checks for an EGR valve that is leaking, stuck open, or turned on at idle. Normal engine operating temperature is specified as 176°F (80°C) or greater.
3. 4 -  This step checks the operation of the EGR valve. The duty cycle output control of the EGR is disabled by the PCM when any DTCs are stored and when the engine temperature is less than 176°F (80°C).
4. 8 -  This step checks for a physically stuck EGR valve or plugged EGR passages.
5. 9 -  This step checks the ignition positive voltage supply to the EGR coils.
6. 10 -  This step checks for proper Manifold Absolute Pressure (MAP) sensor operation. The diagnostic for a DTC P0401 depends on MAP sensor output in order to monitor EGR valve performance.
7. 11 -  Fault not present indicates the condition that caused DTC P0401 to set is intermittent and not currently present. If no other DTCs are stored, refer to DIAGNOSTIC AIDS  for additional information on diagnosing an intermittent DTC P0401.
8. 12 -  This step determines whether an electrical fault is the cause of the EGR valve staying open. A grounded EGR valve control circuit can cause the EGR valve to be open all the time.
9. 14 -  This step checks for an EGR valve control circuit that is shorted to ground. If the circuit wiring is okay, a faulty PCM with an internally shorted circuit can cause the EGR valve to be open.
10. 19 -  Clearing the DTCs by removing the DOME fuse also clears the PCM long term memory. Using the scan tool to clear DTCs leaves the long term memory active. DTC emitting diagnostics may not run to completion as expected unless the long term memory is cleared. Removal of the DOME fuse also clears the time display of the radio. The DTC P0401 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0401 appears in the Not Ran Since Code Cleared list, the DTC P0401 diagnostic has not yet run. When the DTC P0401 does not appear in the Not Ran Since Code Cleared list, the DTC P0401 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0401 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.