New here. Was searching for this issue but did not get clear answers in my search. My car rough idles and fluctuates from 500 to 800rpm when stopped at traffic light. I noticed this happens when AC is on. Right now the temperature in 65F and still has same issue. I am driving through this all summer. Don’t know what is wrong or how to check (no error code on engine)On YouTube someone suggested, update firmware and issue will be resolved. Does that work?

any help would be great

 

If it's happening only when the a/c is on then I don't think that would point to a software issue. I would do a smoke test and test the alternator and battery to see what you get then go from there.

 

Could be low oil pressure (which affects the VANOS system, and results in a lumpy / up-down idle). Very often this is caused by missing O-rings in the "nose" inside your oil filter element (which is inside the oil filter cap). Someone had removed the entire "nose" on mine before I bought it. A new cap and O-rings and it idled like a Swiss watch.

I'm guessing this because the A/C can very slightly reduce the idle speed, which will take marginally low oil pressure down just enough to cause this kind of thing.

 

Thank you. I will check the O rings. Will also google on how to do smoke test. My battery and alternators are about 6 months old