I have a dewalt D55168 air compresser. motor stays running ,air pressure won't go over 35 psi.. thanks , frank n2141s

There is a ground fault in that case. If you are in need to ask this sort of question, I recommend the services of a qualified electrician.

https://www.manualslib.com/manual/494085/Dewalt-D55168.html

Specifncations
MODEL D55168
WEIGHT 88 Ibs. (39.92 kg)
HEIGHT 41.75" (1060.5mm)
WIDTH 19.0" (4826 mm)
LENGTH 21.25 (539.8 ram)
AIRTANK CAPACITY (GALLONS) 15 Gallon (56.8 fiters)
APPROX CUT-_N PRESSURE 160 PSI (1103.2 kPa)
APPROX. CUT-OUT PRESSURE 200 PSI (1379.0 kPa)
SCFM @ 90 PSI (620.5 kPa_ 5._

cant get fixya comment to work so will edit here...

The 200/160 are the typical DS55168 settings from factory and meet the 90 psi @ 5.4 cfm rating.

Pressures can be adjusted to a MAXIMUM 225/185 with a resulting lower cfm rate.

I'm not familiar with "type 9". Likely nothing special just different setting from factory?

You need a new pressure regulator switch.

you are getting much more than 65 psi-pressure gage is not working-cheers Denny

If you bought it at HD return it for a new one. If not you will need to use a meter and check power from receptacle to on/off switch to pressure switch., Check motor winding. If all else fails call Dewalt and they will help you.

Worn ring and cylinder or bad valves maybe.

Check the pressure switch. The unloader valve is actually in the pressure switch where the small air line from the tank check valve attaches to it. When the pressure switch turns the motor off it then lets the head pressure bleed off so the motor can restart the next time pressure drops in the tank. If it's not working and bleeding off the head pressure the cold start valve (the one with the pin in it) also won't open so the motor can restart and warn up properly.

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.

Check for leaks all over the machine. Possibly the O ring on the drain valve is damaged. If the machine is oil-less, the big rubber O ring on the piston could be worn away.
When the machine is turned off can you hear air leaking from anywhere?

The dewalt model d55168 is a high-end oil-free compressor. Oil free compressors could have a short life span due to dusty or dirty conditions at the job site. The cylinder of your compressor is open from the bottom and will **** in debris scratching the cylinder/ silicone-vinyl piston ring reducing pressure. However the reduction in pressure will be over time till the point that the compressor will not reach cut-out pressure and just keep running. If you lost pressure suddenly to "0" then you probably lost the drive belt. A replacement can be found at a local dewalt service center or ordered thru www.toolpartsdirect.com. When you take down the compressor, be sure to examine the cylinder /piston for wear. These are the two most common problems. Good Luck on your repair and email if you have other questions.