Inspiron 3910 Power Failure (just 2 years old)

first do a BIST of the psu

1. Turn off the computer.
2. Press the BIST button and verify if the LED turns on.
3. The LED must be solid and not flickering or flashing. The power supply fan turns on. If the PSU fan does not spin correctly, consider the built-in self-test as failed even if the LED lights up.
   1. If the BIST LED turns on, the power supply is delivering power to the computer.
   2. If the BIST LED does not turn on or the PSU fan does not spin correctly, the power supply is not delivering power to the computer. Disconnect and reconnect the power cable to computer and try again. If the BIST LED does not turn on consider replacing the psu

based on what you reported, LED does not turn on when psu is connected to motherboard (8 and 4 pin).  but you did not say you press the BIST button.  Is there a BIST button at back of your psu?

if there is no BIST button, do the following test

1. Turn off the computer.
2. Disconnect the power cable from the PSU, and wait for 15 to 20 seconds.
3. After 15 to 20 seconds, connect the power cable to the computer.
4. The LED light turns on for 3 seconds and then turns off.
   1. If the BIST LED turns on, the power supply is delivering power to the computer.
   2. If the BIST LED does not turn on or the PSU fan does not spin correctly, the power supply is not delivering power to the computer. Disconnect and reconnect the power cable to computer and try again. If the BIST LED does not turn on, test is failed

I would try a RTC reset of 3910 too.  Press and hold the power button for 30 seconds. The computer RTC reset occurs after you release the power button. NOTE:The RTC reset is aborted if the power button is held for less than 25 seconds or more than 40 second

@redxps630​ My PSU has no BIST button so I use the power switch.  With the CPU power connector plugged in the BIST light never illuminates and the fan never starts when I do the test.  When I disconnect the CPU plug, the LED does come on immediately but the fan does not start. I did the BIST test a few times with the CPU disconnected.  The light goes out for about 35 seconds when the test is in progress. After the test the green light flashes once and goes out.  I am considering replacing the power supply but not optimistic.  

@redxps630​ Thanks for your suggestion. No BIST button on the back of the PSU.  Results explained above. So the test fails but I understand the fault could be in the PSU or motherboard.

@redxps630​ Thanks for your answer. I did not know about the 25 to 40 second window but results were as indicated above.

if you replace psu take this opportunity to consider a higher rating psu if you had originally got the basic 180w psu

@redxps630​ Yes I have just 180W so 300W would be better. Wonder if the PSU is shutting down when the CPU load is added to the 12V rail?  Of course if the mother board is shorting the 12V it won't make any difference.  Dell support seemed to think it was the mother board but they wanted me to send it in for service of course.  

I have a feeling even if you get a good psu replacement it will be same story if the motherboard is bad having a short.  I have two Dell laptop that do the same although they are older than 3910.  one of them was an Inspiron that shorts the power adapter.

replacing 3910 board is costly.  it is unfortunate if the board does this in only 2 years.  we know Inspiron is a low end model line and its component quality is iffy.  I had older Inspiron that still work such as 530, 660, 3650.  It is unpredictable.

I had Dell pc that had POST issue within warranty and Dell did send me a new/refub motherboard by FedEx express (all free) to test replacement at home.  user just have to send back the old supposedly bad board, so Dell made sure user would not keep on to two mobo in case there were fraud. 

if your 3910 is out of warranty ask them how much it costs to send you a test refurb board if possible.  that is if you are willing to repair it.