I was out messing with the new '04 and I did the following
1.) Hold down odometer reset button.
2.) While still holding it down, turn the ignition to the on position but don't start.
After a few seconds the gauges all light up and cycle to full scale and then back to zero, cool. Then the green LED's that normally show your miles read the following:
AC 41 then after a few seconds change to:
00 00
Anyone have any ideas what the codes mean ??
Thanks.

 

The 03's do that as well. It does two things. One, it checks to make sure all of the gauges and lights in the instrument panel are working properly. Two, it will show you the code if you have a problem.