On my 2003 SC430, cold idle is about 1100. warm idle is about 900. Considering all the cars I have owned in over 70 years, this seems high. Most of the threads I read suggest these are normal numbers. Few questions: 1) can I adjust either or both settings down at least 100 to 150 rpms? 2) is there any harm in doing this? 3) Is it really necessary to run so high? (all readings with A/C on) 4) can I adjust them only via the computer, OBD?

 

As far as I know, that is all controlled by the computer. Not user adjustable.

 

i wonder if the throttle body plate is getting stuck open more that it should when its at idle, due to carbon/dirt buildup.
give it a good visual and clean it out with throttle body spray cleaner.

i think there might be a calibration procedure but would need to look that up in the shop manuals or YouTube.

here is a video to watch

 

Somewhere in this forum there are instructions to set the idle speed.
Just keep on searching.

 

The FSM states the idle speed with the A/C off (actually says "cooling fan off", I interpret that to be A/C off) should be 750 RPM +/- 50 RPM.

That should tell you if your car is within the original spec'ed value.