XPS 8940, fans run full speed when shut down, quiet when running

Confirm your power button setting is to turn PC, not put it to sleep.  Go to Control Panel> All  Control Panel Items > Power Options > System Settings and change the power button settings there, if necessary.

If that doesn't help, make sure all your drivers are up-to-date, especially Intel Management Engine.

BTW: What version of BIOS is running?

I pinged my Dell tech contacts...

BIOS  2.3.0

Shut Down for Power Button - Plugged in & on Battery

Sleep button - do nothing plugged in, blank on Battery

Turn on fast startup & Hibernate - unchecked

Checked  do not sleep in the BIOS

Running balanced for performance

Drivers are all up to date

Intel Management Engine up to date, corrected a few errors, but still have same problem...CPU fan & Chassis fan run full speed when computer is shut down, GPU fan not running. Quiet on startup & when it's running normally. Suspect faulty power supply or Motherboard.  99TPH 500w Liteon is the power supply. DELL 0K3CM7 is the motherboard & Intel H470 (Comet Lake PCH-H) is the chipset for it.

I don't know if the Tech's can solve the problem...I think it is an engineering problem. Here is an article I found on line which describes my problem to a tee. "Once the computer shuts down, motherboard is supposed to release the PS_ON pin and let it float. Until it does that, the PSU will remain on and the fans will run at full speed because the fan speed control circuitry is off. It seems that for some reason your PSU does not shut down so the problem is either the power supply which doesn't detect when motherboard releases the PS_ON pin or the motherboard which doesn't release the pin during shutdown." by AndrejaKo. Pass that on to the higher ups, solve the problem, and get back to me. I would like my Dell Outlet reject fixed.

Requested Dell honor the Service Agreement at time of purchase from Dell Outlet which states: "XPS-8940 : 1 Year Hardware Service with Onsite/In-Home Service After Remote Diagnosis." The problem appears to be the motherboard. The tech performed all kinds of tests on Oct 26, 2021 and couldn't determine the problem...finally gave up and said my time had expired. Called back on Dec 24, 2021 and discovered paperwork on their end was in total disarray...still had PC in previous owner's name. All Dell did was to have the PC returned to them and they resold it to me on Dell Outlet as 'Certified Refurbished,' which apparently it wasn't even looked at because I've got the same problem the previous owner had! Problem is as follows: I can plug the computer in after having cooled down for 12 hours, with nothing else plugged in or attached to it, and the CPU and Motherboard (Chassis) fans turn on and run full speed indefinately. I temporally replaced the power supply with known good one with exact same specs, no change! Replaced original. Other than this "Loud noise issue" when PC is shutdown, everything else runs perfect with everything updated and all tests passed according to SupportAssist and dell.com. Here is an article I found on line which describes my problem to a tee. "Once the computer shuts down, motherboard is supposed to release the PS_ON pin and let it float. Until it does that, the PSU will remain on and the fans will run at full speed because the fan speed control circuitry is off. It seems that for some reason your PSU does not shut down so the problem is either the power supply which doesn't detect when motherboard releases the PS_ON pin or the motherboard which doesn't release the pin during shutdown." by AndrejaKo. BOTTOM LINE: 99% sure motherboard needs to be replaced! 'OEM Dell XPS 8940 Desktop Motherboard DW55K K3CM7 10th 11th Gen LGA 1200'. Either have tech come out, verify & replace motherboard, or mail the board to me & I'll replace it myself. Thanks! (Dell has not responded to this email I sent them!)

It may take a bit longer to get a response these days because of holiday vacations/shutdowns, Covid...

I am having this EXACT same issue on same model for a customer. I just finished the urgent bios update (2.4 released Jan 10, 2022) to see if that worked and it fired up w/ high speed fan usage for only a second this time and booted into Windows. Shutting down however still produced full fan speed and did not shut down at all. pushing the power button 1x stops the issue but all that does is start the boot process again. So the urgent BIOS update didn't do anything to remedy this. 

I have had my Dell XPS 8940 for about 8 months.  The problem described with the fan running a high speed after the PC is shut down appeared suddenly.  No changes to hardware, software, etc.  I spent a lot of time emailing tech support and trying things they suggested.  No luck.  They are going to replace the motherboard.  I will update this post when I see the  results of that.  It will probably be next week as I have to be out of town until then.

Tech support has been very sympathetic to my problem and has spent quite a bit of time trying to resolve the issue.

Here's a work-around you might want to try until this gets sorted:

1. Reboot and open BIOS setup
2. Navigate to Power Management >AC Recovery option
3. Change that setting to "Reboot after power failure" (or similar wording)
4. Save the change and exit setup
5. After PC boots, shut down normally
6. Unplug PC from wall
7. Plug PC into power strip or surge protector with its own on/off switch - with that switch off
8. To boot PC, turn the strip/surge on, PC will think it's recovering from power failure and boot. You don't have to touch PC's power button
9. Whenever you shut down normally after that, turn the strip/surge off at its own switch.

My XPS 8930 (set to reboot after power failure), monitor, speakers etc are all plugged into a surge protector with power switch. So just turning one switch on turns everything on at same time, without doing anything else. Just be sure the strip/surge has sufficient capacity to handle the load when devices all turn on at same time.

Also updated BIOS to 2.4.0.  No difference...same problem! It did create an additional problem with the Killer ethernet with a 'Fail' from SupportAssist on both Win 10 Pro & Win 11 Pro. I finally figured that out and both systems are successfully updated according to SupportAssist, Microsoft is updated, & Intel Support assistant (which is more accurate than Dell) is also up to date. Just the Shut Down problem. When the power button light goes out, the cpu & chassis fans kick on high at the exact same time, with the gpu fan off & little or no air from psu fan.

@Dave4967  Have you considered the work-around I posted on page 1 of this thread?

I am puzzled by something in one of your earlier posts:
Shut Down for Power Button - Plugged in & on Battery

Don't know why you would have a "battery" option since this is a desktop PC.  By chance is this PC set to use Tablet mode in Windows. Go to that screen and if not already set this way, change to "Never use tablet mode" and "Don't switch to tablet mode".

I'd also re-enable Fast Startup. Then reboot and see what happens...

It's possible the OS is corrupted and you might may want to image the boot drive and then do a basic clean install of Win 11 including all drivers and Windows Updates, but without any of your own software.

Then see if it shuts off properly now. If not, you can restore your image back onto the boot drive so you don't have to re-install everything again, if the basic clean install doesn't solve the problem.

Macrium Reflect (free) is a good choice to image the boot drive. And be sure to use Macrium, before you do the clean install, to create the bootable USB stick you'll need to restore the image you make.

I tried your work-around by temporarily changing to: 'Restarting after power failure'. No more often than I shutdown, it's easier for me to just unplug the cord at the power supply. (Which by the way should have a real on-off switch next to the power cord.) (Easier to shut off when you get one of their rejects!) The strip/surge is also a battery backup with a usb cable attached to it from the battery backup to the desktop. The pc sees it as a battery...thus the battery icon in power settings. Temporarily tried the Fast Startup...no difference. Haven't done a clean install yet of either win 10 pro or 11 pro, will do that sometime in the near future. (But don't see how the operating system could be a problem. The pc can be shutdown long enough to cool off, & when you first plug it in with nothing else attached to it, same problem with the fans!) Been trying to get a mother board from Dell, but basic tech support keeps lying to me and says they're going to have a tech come out and replace the motherboard. This was suppose to happen tomorrow, but tech support let it fall through the cracks! I know the power supply and the fans are good, because I already temporarily replaced them with known good ones. The only thing left is the motherboard. One thing that's on my mind is cpu motherboard pins...Dell may have a bent or broken one from their poor manufacturing process...they don't make them anymore like they use to!!  

A work order was just created and it included a motherboard. There's still hope yet?! Time still TBD.

Although the info you posted may work Ron, and I appreciate it, this is for a customer and I won't be having them implement that as they can just unplug the cord. This is clearly not normal and I'd like to get it work as so. Unfortunately, their support team from India has disconnected 1x and I kept repeating myself to the other agent. I have zero luck when I call regular support. 

Guess I will monitor Dave and see if a new board was the solution. This customer stated he already sent it in to them and apparently they "fixed" it and replaced these parts, but I don't know how much of that is true.