Studio XPS 435T/9000, motherboard issue

you think it is a bad motherboard?

@redxps630 may be right that the MB is the issue. Before then however pull all of the RAM and see if the system will boot, with an obvious RAM error of course. Then try one RAM stick at a time as it may be the source of the problem. Did you install a brand new CMOS battery or one that you had laying around? You could also try reseating the CPU and repasting the CPU with fresh thermal paste. Make sure there are no bent pins. The next question is was the system working properly previously and had you made any change prior to the problem.

Thanks JO PC was working just a week ago.  Prior to that I had removed/reinstalled oem heatsink and repasted cpu.  battery is new.  I was playing with Win11 install and Dell app install.

Turns out JO is spot on. The heatsink has a plastic ring that holds aluminum heatsink and mounting screws. The mounting screws go through 4 cylinder of the ring and fasten on the motherboard backplate. 2/4 plastic base of the cylinder become detached (fallen out) from the rest of the plastic ring. Although the two screws are still tight on the backplate, they no longer press down on the plastic ring.  By Murphy’s law those 2 screws are next to each other and not diagonal of a 80x80 mm square. So the heatsink are tightened on only two other screws and become lop sided resulting in poor contact with cpu.

this is the stock heat sink, as shown the mounting screws are in 4 plastic cylinders with plastic base.  my heatsink has two of the bases fell out disintegrated from the rest of cylinder.  so the screws are no longer applying mounting pressure on the plastic ring which houses the heatsink.  a design flaw.   a better design is to have screw go through metal cylinder.

@redxps630​ Happy to help out!