One problem related to air conditioner has been reported for the 2003 GMC C2500. The most recently reported issues are listed below.
I have a 2003 GMC 2500 hd duramax diesel. My complaint is the air conditioning doesn't seem to cool very well. I had a 2002 model and it cooled alot better. I am not sure what was change other than this is a dual zone system but is not for the better. I would like if GMC would change it back to the way it was in 2002, especially for those who live in the southern states. I am a professional and when I climb into my truck on a hot day I soak a perfectly good shirt waiting on it to cool down in the cab. This is pathetic to think that GMC's slogan is "professional grade" not as far as the ac!.