SYS.OK|MANUAL.DB● LIVE
HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 89 (ENGINE CONTROLS - SELF-DIAGNOSTICS - 4.0L)DIAGNOSTIC TESTSDTC P0121: THROTTLE POSITION SENSOR CIRCUIT INSUFFICIENT ACTIVITYDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check - Engine Controls? If yes, go to next step. If no, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
    2. NOTE: If any other DTCs are set, diagnose those DTCs first. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
  2. Turn ignition on. Using scan tool, observe TP sensor angle parameter while slowly opening throttle to WOT. Does the TP Angle increase steadily and evenly from zero percent to above 98 percent? If yes, go to next step. If no, go to step  4.
  3. Using scan tool, observe and record FREEZE FRAME/FAILURE RECORDS data for this DTC. Turn ignition off for 30 seconds. Start engine. Operate vehicle within Conditions for Running DTC or as close to recorded FREEZE FRAME/FAILURE RECORDS data as possible. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see DIAGNOSTIC AIDS .
  4. Turn ignition off. Disconnect TP sensor connector. Turn ignition on. Using scan tool, observe TP sensor voltage. Does the scan tool indicate TP sensor voltage at 0 V? If yes, go to next step. If no, to step  6.
  5. Turn ignition off. Connect a fused jumper wire between the TP sensor harness connector 5-volt reference circuit and signal circuit. Turn ignition on, with engine off. With scan tool, observe the TP sensor voltage parameter. Does scan tool indicate TP sensor voltage at 5 volts? If yes, go to step  9. If no, go to step  7.
  6. Using a Digital Multimeter (DMM), test TP sensor signal circuit for short to voltage. Did you find and correct the condition? If yes, go to step  15. If no, go to step  13.
  7. With a DMM, test TP sensor 5-volt reference circuit for an open or high resistance. Did you find and correct the condition? If yes, go to step  15. If no, go to next step.
  8. Test TP sensor signal circuit for an open or high resistance. Did you find and correct the condition? If yes, go to step  15. If no, go to step  12.
  9. Test the TP sensor low reference circuit for an open or high resistance. Did you find and correct the condition? If yes, go to step  15. If no, go to next step.
  10. Test the TP sensor 5-volt reference circuit for high resistance. Did you find and correct the condition? If yes, go to step  15. If no, go to next step.
  11. Check for poor connections at TP sensor harness connector. Did you find and correct the condition? If yes, go to step  15. If no, go to step  13.
  12. Inspect for a poor connection at PCM harness connector. Did you find and correct the condition? If yes, go to step  15. If no, go to step  14.
  13. Replace TP sensor. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. After repairs, go to step  15.
  14. Replace the PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Reprogram PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  15. Use the scan tool in order to clear the DTCs. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step  2.
  16. With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
RENDER: 1.0x

NO RELATED

Recommended Tools & Savings

Use the Manual With the Right Hardware

Pair factory procedures with proven DIY tools so the instructions are easier to execute.

Affiliate disclosure: We may earn a commission at no extra cost to you.

Shop Manual-Ready ToolsFree Quote CheckQuote Shield Pro

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.