XPS 8940, 500W PSU failed

You may try your luck with Dell spare parts at 1-800-372-3355, or 1-800-348-6147

Otherwise, taking your risk at auction sites.  Choose seller with good rep and offering return options.  The part number you provided is also cross reference with G5 5090 and OptiPlex 7080.

You can find your Dell PSU here  . . . you could also jump to 750 watts, 1000 watts, or 1350 watts, if you were so inclined.

Ok thanks ProfessorW00d. Just so I'm understanding, is there a Form/Fit/Function 750W PSU that can replace my Dell P/N Y7R0X 500W PSU? ie, same dimensions, mounting, cable lengths, same connectors, same voltages with higher amps? Which 750W 365powersupply P/N would you recommend to replace my Dell Y7R0X  500W PSU?

Based on further investigation and the response from redxps630 it appears your are maxed at the 500 watt PSU for Dell OEM. The go higher you would need to run a 3rd party unit with some adapter cables.

Re: the green light on the back of the PSU lights-up for 3 seconds then goes out and stays out

not sure that is definite evidence that the 500w psu has failed.

How to run self-test using the button on 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.

Dell oem psu for 8940 max is 500w.  The 750/1000/1350w oem psu are for 8950/8960, which are incompatible with 8940.

8940 mobo has 6+4+4 pin

8950 mobo has 10+4+4 pin

8960 mobo has 12+4+4 pin.

Sorry, but XPS 8940 doesn't have a PSU test button. It only has the green PSU status LED.

If you're using a power strip, surge protector, or UPS, remove those and run the PSU BIST test this way:

How to run self-test without the button on the PSU

1. Turn off the computer.
2. Disconnect the power cable from the PSU, and wait for 15 seconds.
3. After 15 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.

Note the comment in #2 about the PSU fan. So have the PC in a position where you can see the fan inside from the back of the case before you start the BIST.

If the PSU passes the BIST, open the case so you can see the motherboard and then carefully plug PC into the outlet. You should see an LED light up on the board indicting that the PC is getting "flea" power from the PSU. It should be on all the time, as long a PC is connected to a working outlet. Flea power isn't enough to boot the PC, but it's a sign the board can sense power input.

Assuming the flea LED is on, the next thing to consider might be a failed power button module. 

Thanks all for the answers. My XPS 8940 PSU symptoms are from initial AC power applied: Green LED in the back turns on for 3 seconds then goes out and stays out, fan never operates, no LED lit on MOBO.

One other thing I should have mentioned. We had an AC power drop-out (~2 seconds) here in my neighborhood that immediately proceeded the PSU failure. I now believe that was also accompanied with a transient voltage spike. 

I went ahead and ordered the following replacement PSU: Y7R0X 0Y7R0X 500W PSU For Dell G5 5090 7070 7060 D500E005P Power supply H500EPM-00 - 365PowerSupply.com - Replacement Laptop Power Adapters,Desktop Power Supply & Server Workstation PSU.

If you ran the BIST and PSU fan doesn't spin, even if the PSU LED behaved "normally", plus no motherboard LED, does sound like a failed PSU.

Contact your power company asap and ask if they have insurance to cover damages caused by a power outage. Be sure to have date/time of outage, duration etc handy when you contact them.

And if you were using a surge protector and/or UPS, contact those companies to see if they offer any coverage for something like this.

Hope the new one solves the problem....

Ok I received the replacement PSU: Y7R0X 0Y7R0X 500W PSU For Dell G5 5090 7070 7060 D500E005P Power supply H500EPM-00 - 365PowerSupply.com - Replacement Laptop Power Adapters,Desktop Power Supply & Server Workstation PSU.

I installed this last night, plugged in all 3 connectors to the Mobo + 1 to the graphics card. When the line cord is plugged-in, I got no green light at all, not even for a few seconds. Also got no fan operation. The power button on the PC has no effect. So the PC is 'dead in the water'. Any suggestions as to what could be wrong? How can I troubleshoot this?

Sorry to hear that, but something doesn't add up...

You said old PSU lights up the green status LED for a few seconds and goes out, but new PSU doesn't even do that.

Strip PC down to bare essentials. Remove all PCI-e cards including GPU, Disconnect all internal non-boot drives and remove all RAM except the one in 2nd slot from CPU. Remove motherboard battery and press/hold power button for ~30 sec. Reinstall battery, connect mouse, keyboard to rear USB2 ports and monitor to onboard Intel Graphics HDMI or DP. See if it boots now, or at least gives you a green status LED.

If not, reconnect old PSU again, without adding back any other hardware. Then connect mouse, keyboard and monitor, as above.  Will you get the green LED again and does it boot now ?

If old PSU gives green status LED, but doesn't boot, you might want to try shorting the 3 pins for the power button to see if PC will power on.  This is an XPS 8940 motherboard showing power button cable removed and the 3 pins jumped, courtesy of @Tatolino.  You probably will want to practice shorting the pins while PC is unplugged from the mains and then press/hold the power button for ~30 sec before installing the paperclip. When you're comfortable, see if you can get PC to power on and boot with the pins shorted.

Let us know what happens...

If that status LED isn't on the PSU, itself, it's not likely to turn on when the PSU isn't connected to motherboard.

BTW: I've heard the place you bought the replacement PSU is "shady". They sell things for cheap prices that are frequently DOA or fail shortly afterwards. You need to contact them about not seeing the PSU status LED turn on, especially if the old PSU gets a green light...

Ok thanks Ron. I'll give that a try. BTW 1 other thing I had already tried, last night, with the new PSU was to unplug and remove the GPU, disconnect the HDD, and also removed the 1 other PCIe card (extra NIC card). Still got the same behavior. 

I also tried disconnecting all PSU output cables/connectors, then plugged-in the line cord. Still got no green LED lit. Should the green LED light with all the PSU outputs disconnected?

As an act of desperation, I am about to order a refurbished XPS 8940 from Amazon: https://www.amazon.com/gp/product/B09NXBHNHS/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&th=1 

Would it be possible to take the 512GB PCIe M.2 SSD out and 2TB HDD out of my failed XPS 8940 and transplant into this /new' refurbished PC and would it boot? I had Windows 10 Pro on the old machine and that's what I want to continue running. I also had Blue Iris video security S/W on that machine and would like to avoid having to reconfigure everything. Is it overly optimistic to think all this will just come-up and run? BTW I also have a fairly current Acronis image backup of that old machine so that would be another option for this proposed 'brain transplant'. Any recommendations?

Your Windows license on your failed XPS is associated with that specific motherboard, so you would have to use the operating system on the new M.2 SSD. That operating system could be transferred to a different storage device, but the license for that operating system is on the refurbished XPS motherboard. This is my understanding.