Hi 0x0d,

Please try disconnecting the power switch cable from the motherboard, remove the CMOS battery and then connect the power cable to the back of the system. What happens (if anything)? The system should start up normally.

http://topics-cdn.dell.com/pdf/optiplex-9020-desktop_owners-manual_en-us.pdf  (page 20)

If you disconnect the PSU cables from all components to isolate it, does the green LED light up when you push the test button?

Let us know how you get on, thanks.

I already removed the battery as i said but it didn't help

I disconnect the PSU cables from all components and connect the power, it doesn't turn on. I pressed the test button and the LED green light turns on. PSU seems working fine.

I also removed the cable to power button but nope, still turns on and off continuously.

Looks like it's the motherboard then, although the PSU can't be ruled out completely. The only way to be 100% sure would be to swap it with one from another system.

Ok then i'll send it to repair. Thanks for help anyway

I was having the same problem for a long time, but finally i find the real solution to this annoying issue.

First - You need to enter on the BIOS, to do that PRESS F2 at the beginning of all, the you go to POST BEHAVIOR option, normally is at the end of the LEFT BAR/MENU on the BIOS.

Second - MEBx Hotkey must be ON, if it is activated you can continue. Just restart you PC and really quick go to the step 3.

Third - Press Ctrl + (plus) P, you will enter in the Intel Management Engine BIOS Extension, in a normal situation it will ask you for a password, that password is "admin" without the quotes (Factory key).

Fourth - What we are looking for is the "Intel ME Configuration" Option, press enter on it, then they will show you a warning message and you need to say yes "Y" or no "N" with their respective keys.

Fifth - You need to enter on the "Intel ME State Control", once that is done they will show you two (2) options, DISABLE or ENABLE, you need to select DISABLE, once you do that you can exit from yourself (if the PC don`t do it by itself.)

and that it will be all.

@CarlosAMedinaS​ The same issue here. Your method works on my spare desktop dell 9020. Thank you very much. How did you figure it out? Two more tips. First tip, if it turn on and off too promptly and you may has no way to enter the bios (F2), you need remove the bios battery first; of course, you need install it back and power on again; Second tip, for step 3 above, you need provide a new password. The new password has to satisfy many requirements otherwise you will see error. (a, long enough >=8; b, at least one upcast; c, at least one lower case; d, at least one number; e, at least one special like @). Good luck. I almost give up. Feel so luck to see this.