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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 335 (Engine Controls - Self-Diagnostics - 3.0L) >> Diagnostic Tests >> DTC P0638: Throttle Actuator Control Command Performance >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2003 Saturn Vue. 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 .
  2. Observe the DTC information. Are DTCs P0121, P0122, P0123, P0221, P0222, or P0223 also set? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
  3. Turn ignition off. Visually inspect the throttle body for throttle valves that are NOT in the rest position, throttle valves that are binding open or closed, or throttle valves that are free to move open or closed WITHOUT spring pressure. Did you find any of these conditions with the throttle body? If yes, go to step  11. If no, go to next step.
    4. NOTE: Disconnecting the throttle body connector will cause additional DTCs to set.
  4. Disconnect the throttle body harness connector. Turn ignition on, with engine off. Measure the voltage of the Throttle Actuator Control (TAC) motor control 1 and TAC motor control 2 circuits of the throttle actuator motor with a DVOM connected to ground. Is the voltage within 2-4 volts for both circuits? If yes, go to step  6. If no, go to next step.
  5. Is the voltage less than 2 volts for either circuit? If yes, go to step  8. If no, go to step  7.
  6. Turn ignition off. Connect the test light between the TAC motor control 1 and TAC motor control 2 circuit of the throttle body. Observe the test light as you turn ON the ignition. Does the test light flash ON, then turn OFF? If yes, go to step  9. If no, go to step  10.
  7. Test the TAC motor control circuit that measured above the specified value for a short to voltage. Did you find and correct the condition? If yes, go to step  13. If no, go to step  10.
  8. Test the TAC motor control circuit that measured below the specified value for an open or a short to ground. Did you find and correct the condition? If yes, go to step  13. If no, go to step  10.
  9. Test for an intermittent and for a poor connection at the throttle body connector. Did you find and correct the condition? If yes, go to step  13. If no, go to step  11.
  10. Test for an intermittent and for a poor connection at the ECM. Did you find and correct the condition? If yes, go to step  13. If no, go to step  12.
  11. Replace throttle body assembly. After replacing throttle body assembly, go to step  13.
  12. Replace ECM. Perform ECM relearn procedures. See ENGINE CONTROL MODULE  under PROGRAMMING. After replacing ECM, go to next step.
  13. Clear the DTCs with a scan tool. Turn ignition off for 30 seconds. Start engine. Operate vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING DTC . You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? If yes, go to step  2. If no, go to next step.
  14. Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? If yes, diagnose affected DTC(s). See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no DTCs are displayed, system is okay.
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