My '01 idles at 3500rpm right from the start (in Park an Neutral) and does not go any lower. In Reverse and Drive it goes down to about 1600rpm and also does not go any lower. Something is definitely wrong here, check engine light is On, but haven't pulled the codes yet.

So, please list for me all the known reasons why a car would idle high and not go down as the car warms up. Thanks.