I just bought a craftsman 1/2" impact wrench and connected it to my big 11 cfs dual tank compressor - but it will not take the lug nuts off my GMC Yukon. Whats likely the problem?

Pneumatic tools are rated in cfm (cubic feet per minute). Your 1 HP air compressor is rated at probably not more than 3 cfm while the 1/2 " impact driver needs at least 8 cfm, maybe 12 chm of air.
The compressor must provide more chm than tool rating needs.
It is a problem, by the time you look at five horse power air compressors and larger, a lot of electric power is needed. A circuit of 220 volts, 20, 30, or more amps would be required to run those large expensive compressors.
If you can, return the air tool and get a good electric tool instead.
Leave those high air volume air tools to shops that are so equipped to use, or invest if you must.

If the lug nuts are as tight as you say I don't think a 3/8 drive will work. I even ruined a half inch drive by putting a pipe on it. Sears did replace it because it was Craftsman make. Are you near a Canadian Tire store as they usually have Master craft make which is life time guarantee as well.

The two most important specs are CFM at 90 psi (that's how fast the compressor can compress more air when the tank starts to run low) and how big the tank is. Virtually any compressor that has a top pressure rating 20 psi higher than you need for your tool (which is usually about 90 psi) and that has a tank of a couple of gallons will work. But the smaller the tank, the shorter time you can operate the tool before the compressor has to kick in. And the lower the CFM at 90, the longer you will have to wait for the tank to come up to pressure again so you can operate the tool at full pressure. If this is for occasional use and you aren't in a hurry, you don't need much of a compressor at all. If you want to operate continuously, then you should get something with a CFM at 90 psi that is higher than your tool's CFM consumption. Keep in mind that impact wrenches don't operate continuously like paint sprayers do. Good luck. Vote if you found this helpful. Al K

I looked all over,, and the info that I found said that the wrench that you have has a ft' pound rating of 360.

c...

The Chicago Pneumatic air wrench is not the same as the Craftsman, but it's a good brand.
Probably you could sell it if you have no use for it yourself. The impact wrench would have been used to remove lug nuts or other fasteners on an automobile, most likely.

Good luck on deciding what to do with the items.

This gun shouldnt have an adjustment. If you cfm's from your compressor are enough. Too low cfm's mean lower torque. If cfm is high enough for the gun rating then I would say the gun is bad.

Try turning your line air pressure up to at least 90 pounds
Most lug nuts are tightened to 85 pounds or higher

If you plan on using it frequently and for extended amounts of time, you'll want a large one. If it's just for occasional use, a smaller pancake one will probably do, just make sure it has the pressure capability required for the impact wrench.