Does trouble code mean automatic check engine light on?

[Captkaos]

Turning the truck off doesn't erase the codes, disconnecting the battery for at least 30 seconds will erase the codes.

If you want to see what codes are stored, find the ALDL connector (to the right of the parking release) and put a paper clip in the A and B terminals (top right 2 holes) turn the key to on and count the flashes.

It will flash the code 3 times and 12 is always first. The way is lights is light.....pause..... light, light, 3 times. If yours flashes 12 the whole time you have no codes.

you can decode them here;
http://www.troublecodes.net/GM/

44 or 45 would indicate a problem with the O2.