Ac is blowing warm air in my 2005 sierra 2500hd duramax. Any idea

Your system has R-134 Freon in it. the 134 is made up of smaller molecules than the old R-12. The R -134 permeates through the rubber parts of the system. If the system gets too low on Freon, the compressor will stop coming on.(low pressure cut out switch). Have the system serviced with an A.C. machine that can vacuum, measure the amount of Freon in the system, and add the right amount. The machine can also inject die. Later the die will show up under a black light.This will show any leaks you may have.