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Home >> Buick >> 1996 >> Roadmaster Base >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control Systems - Tests W/Codes >> Diagnostic Trouble Codes >> DTC P0400 - EGR System >> Diagnostic Procedures
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Diagnostic Procedures

  1. Perform On-Board Diagnostic (OBD) system check. After performing OBD system check, go to next step.
  2. Start and warm engine to normal operating temperature. Using scan tool, command EGR duty cycle to 100 percent and then back to zero percent. If engine RPM changes (decreases or runs rough) when EGR is commanded to 100 percent, go to next step. If engine RPM does not change when EGR is commanded to 100 percent, go to step 4).
  3. Turn ignition on, with engine off. Using scan tool, review FREEZE FRAME and/or FAILURE RECORDS data and note parameters. Turn ignition off for about 15 seconds. Start engine and operate vehicle within the conditions required for this diagnostic to run, and as close to conditions recorded in FREEZE FRAME/FAILURE RECORDS as possible. Select DTC function then enter DTC P0400. If scan tool indicates that this test failed this ignition, go to next step. If scan tool does not indicate that this test failed this ignition, see DIAGNOSTIC AIDS.
  4. Manually lift EGR valve diaphragm. If engine RPM changes (decrease or run rough), go to next step. If engine RPM does not change, go to step 11).
  5. Disconnect harness at solenoid. Connect vacuum gauge to manifold side of vacuum harness and check vacuum supply to valve. Increase engine speed to about 2000 RPM. If vacuum reading is vacuum at least 7 in. Hg., go to next step. If not, go to step 13).
  6. Turn engine off. Rotate vacuum harness and connect only the manifold side of vacuum harness to solenoid. Connect vacuum gauge to the other port of solenoid. Using scan tool, command EGR solenoid ON and OFF while observing vacuum gauge. If vacuum gauge indicates that vacuum is present when solenoid is on and bleed off when solenoid is off, go to next step. If vacuum is not present, go to step 13).
  7. Reconnect vacuum hoses to solenoid, Using scan tool, command EGR solenoid ON and OFF. If vacuum is indicated when commanded, go to next step. If vacuum is not indicated when commanded, go to step 12). 8)Turn ignition off. Apply vacuum to EGR valve and observe EGR valve diaphragm. EGR valve should move freely and hold vacuum for at least 20 seconds. If EGR valve is free and holds vacuum, go to next step. If EGR valve does not hold vacuum, go to step 14).
  8. Apply 10 in. Hg to EGR valve. Start engine and observe vacuum gauge. EGR should move to the seated position and vacuum should drop while engine is started. If EGR operates as specified, go to step 15). If EGR does not operate as specified, go to step 14).
  9. Repair plugged or restricted EGR passages. After repairs, go to step 16).
  10. Repair plugged vacuum port or vacuum line. After repairs, go to step 16).
  11. Repair or replace faulty vacuum line to EGR valve. After repairs, go to step 16).
  12. Replace EGR solenoid. After replacing solenoid, go to step 16).
  13. Replace EGR valve. After replacing EGR valve, go to step 16).
  14. Check for partially clogged EGR passages, restricted or damaged EGR pipe, or modified or faulty exhaust system components. Repair or replace as necessary. After repairs, go to next step.
  15. Using scan tool, select DTC, CLEAR INFO function. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter DTC P0400. Operate vehicle within conditions for setting this DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool did not indicate that this test ran and passed, repeat step 2).
  16. Using scan tool, select CAPTURE INFO, REVIEW INFO function. If any undiagnosed DTC(s) are displayed, go to applicable DTC test.
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