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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 Performance (3.4L "N" 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. Observing scan tool, depress accelerator to floor, and then slowly release pedal. TP sensor angle value should increase steadily to 100 percent when pedal is depressed and decrease steadily to zero percent when pedal is released. If TP sensor angle value is as specified, see DIAGNOSTIC AIDS . If TP sensor angle value is not as specified, go to next step.
  3. Disconnect TP sensor. Monitor TP sensor reading on scan tool. If TP sensor reading is near zero volts, go to next step. If TP sensor reading is not near zero volts, go to step  5.
  4. Connect a test light between 5-volt reference circuit and signal circuit at TP sensor harness connector. Monitor TP sensor reading on scan tool. If TP sensor reading is 5 volts, go to step  7. If TP sensor reading is not 5 volts, go to step  6.
  5. Check TP signal circuit for short to voltage. Check TP sensor ground circuit for high resistance between PCM and TP sensor. Check TP sensor ground circuit for poor connection. Repair as necessary. After repairs, go to step  9. If circuits are okay, go to step  8.
  6. Check TP sensor signal circuit or 5-volt reference circuit for high resistance between PCM and TP sensor. Check TP sensor signal circuit or 5-volt reference circuit for a poor connection. Repair as necessary. After repairs, go to step  9. If circuit is okay, go to step  8.
  7. Replace TP sensor. After repairs, go to step  9.
  8. Replace PCM. After repairs, go to next step.
  9. Using scan tool, clear DTCs. Turn ignition on, with engine off. Using scan tool, review FAILURE RECORDS data and note parameters. Start engine and operate vehicle as close to conditions recorded in FAILURE RECORDS data as possible. Monitor SPECIFIC DTC info for DTC P0121. If scan tool indicates DTC P0121 FAILED THIS IGN, go to step  2. If scan tool does not indicate DTC P0121 FAILED THIS IGN, system is okay.
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