The 1999 GMC Yukon came with a large, 5.7-liter V-8 engine that pushes out more than 255 horsepower. Generally, large V-8 engines give off a lot of heat. That heat eventually leaves the engine compartment and enters the cockpit through the air vents. This means you need to have a high-powered air conditioning system to cool everyone down during the summer months. If the air conditioning unit is not responding as it should, it is easy to diagnose the problem.
Turn the Yukon on and turn the air conditioning unit on. If the compressor works, move on to Step 2. If the compressor does not turn on, it will need to be replaced. This is a repair that should be handled by a professional technician.