Thanks for the comeback, Drew. I agree with everything you've said. Timing done in bypass, no road test in diagnostic mode, etc.
Problem has always been there since son-in-law bought car, 1 year ago. I have no scanner(s), just the MAC card, P/N ECM-1. Even son-in-law's shop didn't find anyhing with their scanners, except the count appeared to be high.
I'm leaning towards the ECM because of the ohmmeter readings for the control circuit of the IAC. It has two coils and the circuits for both should be the same. And the count is intermittently high, son-in-law told after his shop looked at it, b/c the ECM is trying to correct the idle speed, but, IAC is slow acting due to lack of control, hence, the two different ohm readings between Coil A & Coil B.
The engine does idle, say after selecting DEF or A/C, headlights on, etc., but, after 3-4 minutes.
The control circuit for the coil is a push-pull amplifier. I would think an ohmmeter should read the same values in both directions, I'm just assuming. One cannot find electrical schematics for GM's ECM controllers.