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Home >> GMC >> 1998 >> Forward Control 7.4 B >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Tests W/Codes - 7.4L >> Diagnostic Trouble Codes >> DTC P0101 - MAF Sensor System Performance >> Diagnostic Procedures
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Diagnostic Procedures

  1. Perform POWERTRAIN ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK , then go to next step.
    NOTE: If any MAP related DTCs are set, diagnose affected DTC(s) first before proceeding with test.
  2. Turn ignition on, engine off. Using scan tool, monitor MAP sensor value displayed. If MAP sensor value is greater than 100 kPa, go to next step. If MAP sensor value is not greater than 100 kPa, go to step 20).
  3. Remove MAP sensor from intake manifold. Leave sensor harness connector connected. Using a hand-held vacuum pump, slowly apply vacuum up to 20 in. Hg. Each inch of applied vacuum should result in 3-4 kPa drop in MAP sensor value on scan tool. If MAP sensor value reading changes smoothly through entire range of test, go to next step. If MAP sensor value reading is erratic, go to step 20).
  4. Apply 20 in. Hg to MAP sensor. If scan tool reading is 34 kPa or less, go to next step. If scan tool reading is not 34 kPa or less, go to step 20).
  5. Turn ignition off. Disconnect vacuum source from MAP sensor. If sensor reading returns to original value, go to next step. If sensor reading does not return to original value, go to step 20).
  6. Turn ignition on, engine off. Using scan tool, review and record FAILURE RECORDS DATA. Operate vehicle within FAILURE RECORDS conditions. Monitor SPECIFIC DTC INFO for this DTC. If scan tool indicates that this DTC failed this ignition cycle, go to next step. If scan tool does not indicate that this DTC failed this ignition cycle, see DIAGNOSTIC AIDS.
  7. Check for the following conditions:
    • Objects blocking MAF sensor inlet screen
    • Vacuum leaks in intake manifold, throttle body, EGR flange and pipes, MAP sensor seal, EVAP purge valve seal or fuel meter body seal.
    • Malfunctioning, missing or incorrectly installed PCV.

    Repair as necessary. After repairs, go to step 22). If no problems were found, go to next step.

  8. Turn ignition on, engine off. Using scan tool, observe TP angle. If TP sensor angle is zero percent, go to next step. If TP angle is not zero percent, diagnose using DTC P0121 diagnostic test.
  9. Turn ignition off. Disconnect MAF sensor connector. Turn ignition on, engine off. Check voltage between MAF signal circuit and chassis ground. If voltage is about 5 volts, go to next step. If voltage is not about 5 volts, go to step 11).
  10. Connect test light between MAF ignition feed circuit and ground circuits of MAF sensor connector. If test light illuminates, go to step 14). If test light does not illuminate, go to step 13).
  11. If voltage is less than 4.5 volts, go to step 15). If voltage is not less than 4.5 volts, go to next step.
  12. Turn ignition off. Disconnect VCM connector. Turn ignition on, engine off. Check voltage between MAF signal circuit of VCM connector and ground. If voltage is zero volts, go to step 21). If voltage is not zero volts, go to step 18).
  13. Connect test light between MAF ignition feed circuit of MAF sensor connector and ground. If test light illuminates, go to step 21). If test light does not illuminate, go to step 18).
  14. Check for poor connection at MAF sensor. Repair as necessary. After repairs, go to step 22). If connection is okay, go to step 19).
  15. Check for open, short to ground or short to MAF sensor ground circuit in MAF signal circuit between MAF sensor and VCM. Repair as necessary. After repairs, go to step 22). If circuit is okay, go to step 21).
  16. Locate and repair open in ground circuit between MAF sensor and VCM. After repairs, go to step 22).
  17. Locate and repair open in MAF ignition feed circuit to MAF sensor. After repairs, go to step 22).
  18. Locate and repair short to voltage in MAF signal circuit between sensor and VCM. After repairs, go to step 22).
  19. Replace MAF sensor. After replacing sensor, go to step 22).
  20. Replace MAP sensor. After replacing sensor, go to step 22).
  21. Replace VCM. Program replacement VCM using required equipment. After replacing VCM, go to next step.
  22. Using scan tool, select DTC, CLEAR INFO. Start and warm engine to normal operating temperature. Select SPECIFIC DTC, then enter this DTC. 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).
  23. Using scan tool, select CAPTURE INFO, REVIEW INFO. If any undiagnosed DTCs are displayed, go to applicable DTC test.
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