Idle Air Control (IAC) System Diagnosis)

1. If powertrain OBD system check has been performed, go to next step. If powertrain OBD system check has not been performed, perform powertrain OBD system check. See POWERTRAIN OBD SYSTEM CHECK in TESTS W/CODES article.
2. Turn ignition switch to LOCK position. Connect IAC Driver (J-37027-A) to IAC valve. Set parking brake and block wheels. Turn A/C off. Start engine and let idle. Using scan tool, display RPM. Extend and retract IAC valve using IAC driver. If engine RPM changes, go to next step. If engine RPM does not change, go to step 7).
3. If RPM changes smoothly with each flash of IAC driver between 700 and 1500 RPM, go to next step. If RPM does not change smoothly with each flash of IAC driver between 700 and 1500 RPM, go to step 7).
4. Disconnect IAC driver. Check resistance between appropriate IAC valve terminals. See IAC VALVE RESISTANCE  table. See Fig 1. If resistance is as specified, go to next step. If resistance is not as specified, replace IAC valve and go to step 10).
   IAC VALVE RESISTANCE
   
   

   Terminals No.	Ohms
   1 & 2	40-80
   1 & 4	Infinity
   2 & 3	Infinity
   3 & 4	40-80

5. Check harness between PCM and IAC valve harness connector for opens and shorts. If open or shorts do not exist, go to next step. If open or shorts exist, repair as necessary and go to step 10).
6. Check the following: mechanical malfunction, vacuum leaks, lean or rich mixture, poor connection at PCM, defective PCV valve, A/C compressor circuit for short to ground, poor connection at IAC valve or IAC valve passages for blockage or restrictions. If problem does not exist, go to step 9). If problem exists, repair as necessary and go to step 10).
7. Check IAC passages for blockage or restrictions. If blockage or restrictions do not exist, go to next step. If blockage or restrictions exist, repair as necessary and go to step 10).
8. Replace IAC valve and go to step 14).
9. Replace PCM and go to next step.
10. Connect IAC Driver (J-37027-A) to IAC valve. Start engine. Using IAC driver, extend and retract IAC valve. If RPM does not change as expected, go to step 3). If RPM changes as expected, system is okay at this time.

Fig 1: Identifying IAC & ICM Harness Connector Terminals

Courtesy of GENERAL MOTORS CORP.