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Diagnostic Procedure

WARNING: This page is about a different car, the 2003 Pontiac Grand Prix, 2003 Chevrolet Malibu, and 2003 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
    2. NOTE: Ensure that the Engine Speed parameter stabilizes with each Commanded RPM change to determine if the engine speed stays within 100 RPM of the Commanded RPM.
  2. Set parking brake and block drive wheels. Start the engine. Turn all accessories OFF. Use scan tool RPM CONTROL function to slowly increment engine speed to 1700 RPM, then to 600 RPM, then to 1700 RPM. Exit RPM CONTROL function. Does engine speed stabilized within 100 RPM of the commanded RPM? If yes, go to next step. If no, go to step  4.
  3. Observe FREEZE FRAME/FAILURE RECORDS data for this DTC. Turn ignition switch to OFF position for 30 seconds. Start the engine. Operate vehicle within code enable criteria or as close to FREEZE FRAME/FAILURE RECORDS data that you observed. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see DIAGNOSTIC AIDS .
  4. Turn ignition switch to OFF position. Disconnect IAC valve harness connector. Connect IAC Motor Driver (J 37027-A), or equivalent, to the IAC valve. Start the engine. Use IAC motor driver to command IAC valve in until 600 RPM is reached. Use IAC motor driver to command IAC valve out until 1700 RPM is reached. Return engine speed to desired idle as indicated on scan tool data list. Did the engine speed parameter steadily decrease to near 600 RPM and steadily increase to near 1700 RPM when the IAC valve was commanded in and out? If yes, go to next step. If no, go to step  11.
  5. Connect a test light between one of the IAC valve control circuits and a good ground. Start the engine. Observe the IAC Counts parameter using a scan tool. Command high RPM with the IAC Motor Driver until the IAC Counts parameter starts to increment. Command low RPM with the IAC Motor Driver until the IAC Counts parameter starts to increment. Return the Engine Speed parameter to the Desired Idle Speed parameter. Repeat the above procedure for the other three IAC valve control circuits. Did the test light remain illuminated, never flashing, while the IAC Counts were incrementing for any of the IAC valve control circuits? If yes, go to step  10. If no, go to next step.
  6. Did the test light remain OFF, never flashing, while the IAC counts where incrementing at any of the IAC valve control circuits during the test? If yes, go to step  9. If no, go to next step.
  7. Connect a test light between the IAC coil "A" low circuit and the IAC coil "A" high circuit. Observe the IAC Counts parameter using a scan tool and observe the test light. Command high RPM using the IAC Motor Driver until the IAC Counts parameter starts to increment. Command low RPM using the IAC Motor Driver until the IAC Counts parameter starts to increment. Return the Engine Speed parameter to the Desired Idle Speed parameter. Repeat the above procedure using the test light connected between the IAC coil "B" low control circuit and the IAC coil "B" high control circuit. Did the test light stay illuminated and never flashing while the IAC Counts were incrementing for any of the IAC valve control circuits? If yes, go to next step. If no, go to step  16.
  8. Check for a poor connection at the IAC valve harness connector. Did you find and correct the condition? If yes, go to step  17. If no, go to step  12.
  9. Turn ignition switch to OFF position. Disconnect PCM harness connectors. Using a DMM, test the circuit where test light remained OFF for an open circuit, short to ground, or short to another IAC valve circuit. Did you find and correct the condition? If yes, go to step  17. If no, go to step  12.
  10. Turn ignition switch to RUN position, engine off. Using a DMM, test the circuit where the test light remained ON for the following: A short to voltage, or short to another IAC valve circuit. Did you find and correct the condition? If yes, go to step  17. If no, go to step  12.
  11. Visually/Physically inspect for the following:
    • Throttle body damage and tampering.
    • Throttle lever screw tampering (if equipped).
    • Restricted air intake system. Check for a possible collapsed or clogged air intake duct before and after air filter element, restricted air filter element, or restriction at throttle body intake screen, if equipped.

    Remove IAC valve. Check for the following:

    • A clogged IAC passage.
    • Excessive deposits on throttle plate.
    • Excessive deposits in throttle bore.
    • Excessive deposits on IAC valve pintle.

    Did you find and correct the condition? If yes, go to step  17. If no, go to step  14.

  12. Check for a poor connection at the PCM harness connectors. Did you find and correct the condition? If yes, go to step  17. If no, go to next step.
  13. Disconnect PCM harness connectors. Using a DMM, test all IAC valve circuits for high resistance. Did you find and correct the condition? If yes, go to step  17. If no, go to step  16.
  14. Check for a poor connection at the IAC valve harness connector. Did you find and correct the condition? If yes, go to step  17. If no, go to next step.
  15. Replace IAC valve. When complete, go to step  17.
  16. Replace PCM. See POWERTRAIN CONTROL MODULE in appropriate REMOVAL & INSTALLATION article. Perform PCM relearn procedure. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After replacing PCM, go to next step.
  17. Using scan tool, clear the DTCs. Turn ignition switch to OFF position for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running Test. See CONDITIONS FOR RUNNING TEST . 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.
  18. Using the scan tool, observe the stored information in 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 at this time.
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