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Home >> GMC >> 1999 >> Forward Control 6.5 F, Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System - Tests W/Codes - DTC P0100-P0134 >> DTC P0121 >> DTC P0121: TP Sensor Circuit-Insufficient Activity (5.7L "F" Body) >> Diagnostic Procedures

Diagnostic Procedures

  1. Perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS INTRODUCTION article. After performing OBD system check, go to next step.
  2. Start and idle engine. Using scan tool, monitor FAILED THIS IGNITION option under DTC information option. If DTC P1639 fails this ignition cycle, diagnose DTC 1639. If DTC P1639 did not fail this ignition cycle, go to next step.
  3. Start engine. Monitor MAP sensor voltage. Increase engine speed to 2000 RPM. If MAP sensor voltage changed when engine speed changed, go to step  8. If MAP sensor voltage did not change when engine speed changed, go to next step.
  4. Turn ignition on, engine off. Move/wiggle MAP sensor and PCM harness connectors. If MAP value changes when harness or connectors are moved, go to step  16. If MAP value does not change, go to next step.
  5. Remove MAP sensor from intake manifold. Leave harness connector connected. Install a hand-held vacuum pump to MAP sensor. Slowly apply vacuum (up to 20 in. Hg) to MAP sensor. Each inch of vacuum applied should result in a 3-4 kPa drop in MAP sensor value on scan tool. Observe if value changes smoothly with each increase in vacuum. If MAP value changes smoothly, go to next step. If MAP value does not change or is erratic, go to step  17.
  6. Apply about 20 in. Hg to MAP sensor. If MAP reading is 34 kPa or less, go to next step. If MAP reading is not 34 kPa or less, go to step  17.
  7. Disconnect vacuum source from MAP sensor. If MAP reading returns to original value measured in step 5)  , go to next step. If MAP reading does not return to original value, go to step  17.
  8. Turn ignition on, engine off. Monitor TP sensor voltage while moving accelerator from closed to wide open throttle. TP sensor reading should change from less than one volt to greater than 4 volts. If TP sensor reading is as specified, go to next step. If TP sensor reading is not as specified, go to step  10.
  9. Review and record FREEZE FRAME and/or FAILURE RECORDS data for this DTC. Turn ignition off for 15 seconds. Start engine. Operate vehicle within conditions required for diagnostic run, and as close to conditions recorded in FREEZE FRAME/FAILURE RECORDS as possible. Using scan tool, select DTC option, SPECIFIC DTC option, and then enter DTC which was set. If scan tool indicates that this diagnostic failed this ignition cycle, go to next step. If scan tool does not indicate that this diagnostic failed in this ignition cycle, see DIAGNOSTIC AIDS .
  10. Disconnect TP sensor harness connector. Observe TP sensor display on scan tool. If TP voltage is zero volts, go to next step. If TP sensor is not zero volts, go to step  13.
  11. Using a fused jumper wire, jumper TP sensor 5-volt reference circuit to TP sensor signal circuit. If TP sensor voltage is about 5 volts, go to next step. If TP sensor voltage is not about 5 volts, go to step  14.
  12. Replace TP sensor. After repairs, go to step  18.
  13. Check TP sensor signal circuit for short to voltage, TP sensor ground for high resistance between PCM and TP sensor, or TP sensor ground circuit for poor connection. Repair as necessary. After repairs, go to step  18. If circuits are okay, go to step  15.
  14. Check TP sensor signal circuit or 5-volt reference circuit for poor connection. Check TP sensor signal circuit or 5-volt reference circuit for high resistance between PCM and TP sensor. Repair as necessary. After repairs, go to step  18. If circuits are okay, go to next step.
  15. Replace PCM. Program replacement PCM using required equipment. After repairs, go to step  18.
  16. Locate and repair affected circuit. After repairs, go to step  18.
  17. Replace MAP sensor. After repairs, go to next step.
  18. Using scan tool, clear DTCs. Start and warm engine to normal operating temperature. Select DTC option and SPECIFIC DTC option. Operate vehicle within conditions for running the DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool indicates that this test did not pass, go to step  2.
  19. Select CAPTURE INFO option and REVIEW INFO on scan tool. If scan tool displays any DTCs that have not been diagnosed, diagnose affected DTCs. If no undiagnosed DTCs are present, system is okay.
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