An air conditioner in a car, unlike one in the home, must operate while the car is idling or in motion. Vibrations from the car's motor, along with an influx of dirt and other contaminants from the road, can cause problems with the air conditioner's operation. Troubleshoot your GMC air conditioner before taking it to the mechanic for what can often be a costly diagnostic fee.
Pop open the hood of the car. Play the beam of a flashlight against the refrigerant hoses that are located next to the AC's compressor. Holes that have developed in the refrigerant hoses will cause the car's air conditioner (AC) to put out less cold air than it should. Any oily residue that you see indicates lubricating oil, which is present in the system, has escaped through a hole in the hose. Close the hood and drive the car to a car dealership or after market car repair shop to have the hose or hoses replaced.
A total shutdown of the GMC car's air conditioner indicates a loss of electrical power as the result of a blown fuse. Locate the trim panel beneath and to the left of the steering wheel on the underside of the car. Open the driver's door to provide more room for maneuvering. Insert the tip of a flat-edge jeweler's screwdriver into the seam of the right side of the trim panel that is covering the fuse panel. Lift up on the tip of the screwdriver to loosen the trim panel. Pull the trim panel off. Pull the fuse from the air conditioner's fuse socket out with your fingers--the manual that comes with the car will indicate which fuse to pull. Dispose of the fuse in the trash. Place the replacement fuse in the fuse socket. Replace the trim panel.
A loose fitting from the instrument panel to the air conditioner's internal fan can cause intermittent loss of power. Insert the tip of a flat-edge jeweler's screwdriver into the seam on the left side of the bezel of the trim plate surrounding the car's instrument panel. Pull the bezel off. Remove the screws surrounding the instrument panel with a jeweler's screwdriver. Pull the instrument panel out of the dashboard. Pull the electrical connector plug out of the back of the instrument panel. Blow out the electrical connector socket on the instrument panel with a burst of compressed air. Shake the can of electrical cleaner spray. Spray a two second burst into the socket. Plug the electrical connector plug back into the socket. Reverse the disassembly of the instrument panel to reassemble it into the dashboard.