Goodman GMS95 Furnace. New and sometimes does not turn on. It is a forced draft, condensing type unit. Trouble shooting light indicates bad pressure switch. Have replaced both pressure switches. When starting to remove top cover unit would start and run fine.
What else could be keeping this unit from running if pressure switches are good?
Nothing in the manual's trouble-shooting guide seems to be the problem.

What did you mean ''starting to remove the top cover''? Does the unit go clear dead when it won't run? If so, it could be a misalined switch that should cut the power when you remove the cover. If the furnace will run with you holding the switch in, study how to make the cover hold it in.

Did the installers run a fresh air pipe to the outside? If so it may have a spider web or wasp nest in it. Since this furnace is sealed combustion if either flue or fresh air have any restrictions you will get a pressure switch failure code. I use a golf ball to clear then out. Check how the pipes are run and where the ball will exit at the furnace before you try it though. Also many times I see installers use to many elbows or to long a pipe or even the wrong size pipe on installs, it may work for 1 season then it's nothing but troubles. There is a table for the flue pipes inside the installers manual. Be sure to seal the flue again with silicone if you have to break it loose.