I have a D55168 Dewalt compressor. Since I've had it, it has never shut off automatically and I've always shut it off manually at 200 psi. If I leave it alone, will it automatically shut itself off at a pressure greater than 200psi, or do I have a problem? I'm affraid to let it run beyond the 200psi level to see what happens!

Your concern is very well placed, at pressures that high a lot of damage can be caused by a failure. According to the manual for this model air compressor the approximate cut-in pressure is 160 psi and the approximate cut-out pressure is 200 psi. Meaning it should pump up to 'about' 200 psi and shut off and not come on again until it gets down to 'about' 160 psi. They say approximate and I say about because it's not an exact science setting the pressure switch. Spring rates are not always exact, pressure guages are not that precise, etc. When turned on, it should shut off at 200 psi +/- 10 lbs. If it goes more than 10 lbs over that (220psi), I'd recommend you get the pressure switch and or guages checked or replaced.