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Home › GMC › 1996 › C3500 HD 6.5 F, Automatic › Repair and Diagnosis › Engine Performance › System › Engine Controls - Tests W/Codes - Gasoline › Diagnostic Trouble Code Tests › DTC P0401 - EGR System Performance › Diagnostic Procedures

Diagnostic Procedures

  1. Perform On-Board Diagnostic (OBD) System Check. After performing OBD system check, go to next step.
  2. Install scan tool. Start engine. Using scan tool, select MISCELLANEOUS TESTS, EGR CONTROL and cycle EGR valve through 25, 50, 75 and 100 percent positions. Increase engine speed to greater than 1500 RPM. RPM should drop and engine should run rough as EGR valve is cycled. If engine runs rough as EGR valve is cycled, go to step 5). If engine does not run rough as EGR valve is cycled, go to next step.
  3. Turn ignition off. Disconnect EGR valve connector. Turn ignition on, with engine off. Connect test light between EGR control circuit and ignition feed circuit of EGR valve connector (engine harness side). If test light illuminates, go to next step. If test light does not illuminate, go to step 7).
  4. Start engine. Using scan tool, select MISCELLANEOUS TESTS, EGR CONTROL and cycle EGR valve. With test light still connected, observe test light as EGR valve is cycled through 100 percent. If test light illuminates, go to step 8). If test light does not illuminate, go to step 11).
  5. Remove EGR valve and inspect for restrictions. Inspect if valve pintle is sticking partially open. If a problem was found, go to step 10). If a problem was not found, go to next step.
  6. See DIAGNOSTICS AIDS.
  7. Check for short to ground in EGR control circuit. If a problem was found, go to step 14). If a problem was not found, go to step 15).
  8. Check for a restriction in EGR tube or passage. If a problem was found, go to step 10). If a problem was not found, go to next step.
  9. Check for poor connection at EGR valve. If a problem was found, go to step 14). If a problem was not found, go to next step.
  10. Replace EGR valve. Go to step 16).
  11. Check for open in EGR control circuit. If a problem was found, go to step 14). If a problem was not found, go to next step.
  12. Check for open in ignition feed circuit. If a problem was found, go to step 14). If a problem was not found, go to next step.
  13. Check for poor connection at VCM. If a problem was found, go to next step. If a problem was not found, go to step 15).
  14. Repair circuit as necessary. Go to step 16).
  15. Replace VCM. Program replacement VCM using required equipment. After repair is completed, go to next step.
  16. Using scan tool, select DTC, CLEAR INFO. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter DTC P0401. Operate vehicle within the conditions for setting this DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool does not indicate that this test ran and passed, repeat step 2).
  17. Using scan tool, select CAPTURE INFO, REVIEW INFO. If any undiagnosed DTC(s) are displayed, go to applicable DTC test.
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