Yes disconnect ground on battery for a day or two. If the codes come back go to picknpull and get a used iacv and slap it on. You already disconnected the iacv? Was the connector melted or deformed? I’m curious how many miles on the g20? You should also change the cabin air filter sounds like it might be filthy. Make sure you don’t have vacuum leaks the rubber hoses on these get brittle after so many years.
It's got 135,000 miles on it but no engine problems I can see. I can't believe I got it for only a grand!!

The biggest problem is cosmetics, driver side door got side swiped in a hit an run which previous owner chose not to repair. So driver side window won't roll down. Computer always thinks all the doors are open because of this.
Besides the IACV situation, I just got change out driver side door, repaint it (super bad right now) and do other minor things (rear light broken, needs brakes, rotors, shifter gets stuck in park, etc). Runs really well besides a slightly higher idle. Already changed out the cabin air filter (there was like piles of dirt in the cabin).
I haven't disconnected the IACV yet, I just cleaned the connector and the little air way like they show in this video
(did everything exactly the way this guy did in the video).
Like I said, I was worried about disconnecting the IACV because others have reported they had to re-calibrate it at the dealer afterwards and I can't afford dealer service plus its a pain to take the entire thing out and I haven't bought a new one yet. Thanks for the tip. I'll check the hoses, maybe I should just order new hoses for everything?