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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 51 (Engine Control System - Self-Diagnostics - 4.2L) >> Diagnostic Tests >> DTC P1221: Excessive Difference Between Throttle Position Sensors >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2002 Oldsmobile Bravada, 2002 GMC Envoy XL, 2002 GMC Envoy, and 2002 Chevrolet TrailBlazer. 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, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Using a scan tool, observe the DTC information. Is DTC P1635 or P1639 also set? If yes, see DTC P1635: 5-VOLT REFERENCE A CIRCUIT  or DTC P1639: 5-VOLT REFERENCE B CIRCUIT  . If no, go to next step.
  3. Turn ON the ignition, with the engine OFF. Monitor the TP sensors Agree/Disagree parameter using the scan tool. Does the scan tool indicate Disagree? If yes, go to step  6 . If no, go to next step.
    4. NOTE: Removing the ETC fuse will set additional DTCs.
  4. Remove the resonator assembly from the throttle body assembly. Remove the ETC fuse. Open the throttle blade to wide open throttle (WOT) by hand and then return it to closed throttle. Stop briefly at approximately 25 percent, 50 percent, 75 percent and 100 percent in each direction. Test the Throttle Sensors Disagree parameter at each stop. Does the scan tool indicate Disagree while rotating the throttle blade? If yes, go to step  6 . If no, go to next step.
  5. Observe the Freeze Frame/Failure Records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running DTC or as close to the Freeze Frame/Failure Records data that you observed. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM.
  6. Disconnect the TP sensor electrical connector. Turn ON the ignition, with the engine OFF. Measure the voltage between the 5-volt reference circuit of TP sensor 1 and the TP sensor 1 low reference circuit with a Digital Multimeter (DMM). Measure the voltage between the 5-volt reference circuit of TP sensor 2 and the TP sensor 2 low reference circuit with a DMM. Does the DMM display a voltage 4.9-5.1 V for both measurements? If yes, go to step  8 . If no, go to next step.
  7. Test the voltage on the 5-volt reference circuit for TP sensor 1 and TP sensor 2 to a battery ground with a DMM. Does the DMM display a voltage 4.9-5.1 V? If yes, go to step  10 . If no, go to step  9 .
  8. Observe the TP sensor 1 and TP sensor 2 angle on the scan tool. Does the scan tool indicate both sensors at 100%? If yes, go to step  12 . If no, go to step  11 .
  9. Test the 5-volt reference circuit of the TP sensor 1 and TP sensor 2 for high resistance. Did you find and correct the condition? If yes, go to step  15 . If no, go to step  13 .
  10. Test the low reference circuit of the TP sensor 1 and TP sensor 2 for high resistance. Did you find and correct the condition? If yes, go to step  15 . If no, go to step  13 .
  11. Test the signal circuit of the TP sensor 1 and TP sensor 2 for high resistance. Did you find and correct the condition? If yes, go to step  15 . If no, go to step  13 .
  12. Replace the throttle body assembly. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. After repairs, go to step  15 .
  13. Test for poor connections and/or terminal tension at the TP sensor and at the PCM. Did you find and correct the condition? If yes, go to step  15 . If no, go to next step.
  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. Clear the DTCs with a scan tool. 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. Observe the stored information, Capture Info with a scan tool. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS  . If no, system is okay.
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