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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 P0121 - TP Sensor Performance >> Diagnostic Procedures
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Diagnostic Procedures

  1. Perform POWERTRAIN ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK , then go to next step.
  2. Install scan tool. Turn ignition on, with engine off. Using scan tool, observe TP sensor value. If TP sensor value is greater than 2 percent, go to step 4). If TP sensor value is not greater than 2 percent, go to next step.
  3. Depress throttle while observing scan tool. Throttle value should increase from about zero percent to 99.6 percent. Release throttle. Throttle value should drop to about 2 percent. If TP sensor value is greater than 2 percent, go to next step. If TP sensor value is not greater than 2 percent, go to step 5).
  4. Disconnect TP sensor connector. Using scan tool, monitor TP sensor. If TP sensor voltage is about zero volts, go to step 6). If TP sensor voltage is not about zero volts, go to step 9).
  5. DTC is intermittent. If no additional DTCs are set, see DIAGNOSTIC AIDS. If any additional DTCs are set, go to applicable DTC test.
  6. Check for poor connection at TP sensor. If a problem is found, go to next step. If no problem is found, go to step 8).
  7. Repair as necessary. After repairs, go to step 10).
  8. Replace TP sensor. After replacing sensor, go to step 10).
  9. Replace VCM. After replacing VCM, go to next step.
  10. Using scan tool, select DTC, CLEAR INFO. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter this DTC. Operate vehicle within the conditions for setting this DTC. If scan tool indicates this test ran and passed, go to next step. If scan tool does not indicate this test ran and passed, repeat step 2).
  11. Using scan tool, select CAPTURE INFO, REVIEW INFO. If any undiagnosed DTCs are displayed, go to applicable DTC test.
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