There are many threads with lots of instructions on how to get the codes using an LED. There is a thread with a link to a fellows site where he has a simple diagram about how to make a test box with an LED, some test clips and a few passive devices. I got all the stuff to make one at Radio Shack for about $20 and made one. It works quite well.

You can just rig up an LED without putting together the box, but the box has proven quite handy.

I would do a forum search on climate control diagnostic or some such until you find it.

Good luck,