Had the battery change which triggered the auto-theft device on the head unit. As many have stated before simply putting the correct code in is not suffcient. Unfortunately you will need to disconnect the antenna plug from the back of unit THEN enter the code. Worked like a charm. I know other ppl have had luck doing other things, but this WITHOUT a doubt will work.

Here's a picture diagram on how to taking apart the knee panel. http://www.skyeflye.com/acura_cls/In...01-2003_CL.pdf