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Home >> Chevrolet >> 1998 >> Prizm Base, Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Tests W/Codes >> Diagnostic Trouble Codes (DTCS) >> DTC P0121: Throttle Position (TP) Sensor System Performance >> Diagnosis & Repair

Diagnosis & Repair

  1. Perform On-Board Diagnostic (OBD) System Check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK. After performing OBD system check, go to next step.
  2. Connect scan tool to DLC. Turn ignition on and engine off. Using scan tool, select TP sensor angle parameter. Monitor scan tool while depressing accelerator pedal several times. If TP sensor value varies steadily between 10 percent (released position) and 80 percent (depressed position), go to next step. If TP sensor value is not as specified, or reading is erratic, go to step 4).
  3. Turn ignition on with engine off. Using scan tool, clear DTC information. Using scan tool, place vehicle into check mode. Operate vehicle within freeze frame conditions. If DTC P0121 resets, go to next step. If DTC P0121 does not reset, go to DIAGNOSTIC AIDS. Check for an intermittent problem. See TESTS W/O CODES article.
  4. Disconnect TP sensor electrical connector. Using scan tool, select TP sensor angle. If TP sensor value is greater than zero percent, go to step 6). If TP sensor value is zero percent, go to next step.
  5. Connect a test light between TP sensor connector (harness side) 5-volt reference voltage circuit and TP sensor input circuit. See WIRING DIAGRAMS article. Using scan tool, monitor TP sensor angle. If TP sensor value is greater than 90 percent, go to step 8). If TP sensor value is less than 90 percent, go to step 7).
  6. Check for a short to battery voltage in TP sensor input circuit. Check for an open in TP sensor ground circuit between TP sensor and PCM. Check TP sensor electrical connector for proper connection. Repair as necessary. After repairs, go to step 11). If circuits and connection are okay, go to step 10).
  7. Check for an open or short in TP sensor input circuit. Check for high resistance in 5-volt reference voltage circuit between TP sensor and PCM. Repair as necessary. After repairs, go to step 11). If circuits are okay, go to step 10).
  8. Check TP sensor adjustment. See THROTTLE POSITION (TP) SENSOR in ADJUSTMENTS article. Repair as necessary. After repairs, go to step 11). If adjustment is okay, go to next step.
  9. Replace TP sensor. After repairs, go to step 11).
  10. Replace PCM. After repairs, go to step 11).
  11. After repairs, using scan tool, clear DTC information. Road test vehicle within freeze frame conditions that set DTC. Check for DTCs. If no DTCs are stored, repair is complete. If any DTCs are set, diagnose DTC(s).
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