Dell Slim Desktop Optiplex 790

Hello Sir

I already replaced the CMOs battery but nothing happen.

Replacing  battery does not repair a dead power supply

What is the Power Supply Unit (PSU)

built-in self test (BIST)

https://www.dell.com/support/kbdoc/en-us/000125179/

https://www.youtube.com/watch?v=NqeYztUZaf4

Have you already tried:

To reset the BIOS/CMOS, remove the coin-cell battery located on the motherboard, wait 5 minutes, then reinstall the battery

Does the fan spin on and off by itself continuously after you press power button?  If you remove all the memory sticks, does pc beep after pressing power button?

Hi

yes i try it, When you plug in the power cable the fan it is automatic on after a few seconds is going stop then again. 

Re: When you plug in the power cable the fan it is automatic on after a few seconds is going stop then again. 

that by itself is a normal behavior of optiplex.

If you press power button and pc does not start at all, that is abnormal.  there should be a steady organge led on motherboard all the way when power is plugged in.  If reset CMOS makes no difference, try removing ram stick to see if pc beeps.  no beep when pressing power is a bad sign.  try a different working psu can rule out psu problem.

Hi

PSU has been replaced already, the problem still the same. 

looks like motherboard is bad after these diagnostic tests (resetting CMOs and replacing psu).  I bet the cpu is stone cold if you test system without heatsink.  

Hello Sir

Not  yet solve the issue, As per my observation there is shorted a components like  mosfet or capacitor.