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Home >> Buick >> 2002 >> Century Custom >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System Self-Diagnostics - 3.1L Engine >> Diagnostic Tests >> DTC P0404: EGR System >> Diagnostic Procedures
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Diagnostic Procedures

  1. Perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
    NOTE: If DTC P0403 or P0405 is set, diagnose affected DTC(s) first. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
  2. Install a scan tool. Turn ignition on, with engine off. With a scan tool, command the EGR valve from 0 to 100 percent. Does the EGR position sensor remain within 15 percent of the Desired EGR Position at all commanded positions? If yes, go to next step. If no, go to step  4.
  3. Observe the Freeze Frame/Failure Records data for this DTC. Turn ignition off for 30 seconds. Operate the vehicle within the Conditions for Running the DTC or as close to the Freeze Frame/Failure Records data that you observed. See CONDITIONS FOR RUNNING DTC . Does the DTC fail this ignition? If yes, go to next step. If no, go to DIAGNOSTIC AIDS .
  4. Turn ignition off. Disconnect the EGR valve. Turn ignition on, with engine off. With a scan tool, observe the EGR position sensor parameter. Does the scan tool indicate that the EGR position sensor is at 0 percent? If yes, go to next step. If no, go to step  8.
  5. Measure the voltage from the 5-volt reference circuit of the EGR position sensor to the low reference circuit of the EGR position sensor. Does the Digital Multimeter (DMM) display voltage about 5 volts? If yes, go to next step. If no, go to step  7.
    NOTE: If the fuse blows, locate and repair a short to ground in the signal circuit.
  6. Connect a fused jumper between the 5-volt reference circuit of the EGR position sensor and the EGR valve position signal circuit of the EGR position sensor. Does the scan tool indicate that the EGR position sensor is about 100 percent? If yes, go to step  12. If no, go to step  10
  7. Measure the voltage from the 5-volt reference circuit of the EGR position sensor to a good ground. Does the DMM display voltage about 5 volts? If yes, go to step  9. If no, go to step  11.
  8. Test the EGR valve position signal circuit for a short to voltage. Did you find and correct the condition? If yes, go to step  16. If no, go to step  13.
  9. Test the low reference circuit for an open or high resistance. Did you find and correct the condition? If yes, go to step  16. If no, go to step  13.
  10. Test the EGR valve position signal circuit for high resistance or a high resistance short to ground. Did you find and correct the condition? If yes, go to step  16. If no, go to step  13.
  11. Test the 5-volt reference circuit of the EGR position sensor for an open, a short to ground, or a short to voltage. Did you find and correct the condition? If yes, go to step  16. If no, go to step  13.
  12. Inspect for poor connections at the harness connector of the EGR valve. Did you find and correct the condition? If yes, go to step  16. If no, go to step  14.
  13. Inspect for poor connections at the harness connector of the PCM. Did you find and correct the condition? If yes, go to step  16. If no, go to step  15.
  14. Replace the EGR valve. After repairs, go to step  16.
  15. Replace PCM. Perform PCM relearn procedure. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After replacing PCM, go to next step.
  16. Use the scan tool in order to clear the DTCs. Turn ignition off for 30 seconds. Start engine. Operate the vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING DTC . Does the DTC run and pass? If yes, go to next step. If no, go to step  2.
  17. Using scan tool, observe stored information and Capture Info. If scan tool displays any undiagnosed DTCs, diagnose affected DTCs. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no other DTCs are displayed, system is okay.
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