2002 GMC Sierre Crewcab 4x4 Duramax Diesel A/C problem on 2003 duramax, air blower blows but air is not always cold. Often starts blowing cold air but the air gets gradually warmer as if AC is not on. I can actually turn AC off and circulate air in cab and get cooler air that way. It has been checked by gauge to have plenty of coolant.... any suggestions for testing compressor or another part?

Ok now you said,the system was checked to be sure the refrigerant level was full . It could also be over full causing a over preasure shutdown. The compressor has a low and high pressure switch, that will disengage the compressor clutch if it senses a over or under preasure situation to protect the system.Did you check to see if the compressor clutch was Engaging and will it stay Engaged or disengage and keep cycling like that? you Will need to get the preasures on the high and low side of the compressor from a gauge manafold set and please record the outside temp. it will change the preasures in the a/c system than i can better help with the issue you have. also a/c will always work better if recirculated air is used, because it is already cooler than out side super heated air.

my name is ron and if you get the preasures for me i will be glad to check the preasures to My R-134 refrence card