I have spent so may hours researching this problem, and for two years this check engine light has stayed on, my car idled rough, to high and and just never sounded smooth, I have replaced a perfectly good AOS, tried 5 different mass air flow sensors; had the car smoke tested, put on pwis and no luck. Reset the code and two days later, hello little yellow light. What really perplexed me is if you un plugged the mass air it idled great so it must be the mass!! tried 5 different ones and then two days later......... Yesterday I went to my buddies mikes shop and we put the car on the computer and set about looking for a vac leak. I watched the readings as mike checked every place by spraying brake cleaner on anything that carried vac. All through the test the trim stayed around + 25 rich. Then just by chance mike said " lets see if you have a leaf on your air box screen". Mike pulled the filter while the car was running and Bingo the idle dropped and smoothed out and the computer showed trim fluctuating between 2% lean and 2% rich Perfect. push the filter in and 25 % rich . This made no sense at all . It was then we I noticed the frame of the filter holder something was wrong it seemed some thing was missing? I looked on pet and sure enough there are three parts ,i had two. Sometime ago, somewhere ,I guess when my filter was being checked the sliding tray had been lost. It was probably mistakenly thrown out with the old filter! It may have been me I don't know (you can't cure stupid!!) It still made no sense that a motor that is too rich with the air cleaner out , it should be lean. I went to a dance with my wife and all night i thought about this. It does make sense if you think about it, the filer is not sealing in the air box, then the computer would add fuel to compensate for the extra air. I went this morning to my buddies Porsche bone yard and got the filter Assembly . I installed it and wallah perfect idle. Also yesterday we noticed my expansion tank was starting to leak , and getting the car hot on the hoist finished it off . I guess if an expansion tank is going to go it may as well be on a hoist!! maybe the car was so happy we fixed it it peed itself. Last night on the way to the dance we got stuck in traffic on the 401 for almost two hours. We arrived late then won the door prize !! so a good day or bad