XPS 8950, BIOS 1.17.1 broke 1/2 my memory

To be clear, did you try each module alone in 2nd slot (DIMM1) from CPU on motherboard?

With only two matched modules, they should be in 2nd slot (DIMM1) and 4th slot (DIMM2) from CPU. 

You could try resetting BIOS, it case it's just not seeing that one module:

1. Reboot and start tapping F2 when you see Dell splash screen to open BIOS setup
2. Copy down all current BIOS settings, to be safe
3. Power off, unplug power cord from rear of PC
4. Press/hold power button for ~15 sec
5. Open case and remove motherboard battery (check Service Manual for details)
6. Press/hold power button for ~30 sec
7. Install RAM in 2nd and 4th slot from CPU
8. Reinstall battery
9. Close up and connect mouse, monitor and keyboard
10. Reboot and open BIOS setup again. Make sure the current settings match the ones you copied, especially the SATA Operation setting. Save any changes you may make and exit. PC should automatically boot to desktop

Yes, tried each module alone in 2nd slot (DIMM1) from CPU on motherboard, only one DIMM will boot.

As far as resetting the BIOS, tried that. I did everything you suggest, only one DIMM will boot.

I'm an old' HP tech, so that is stuff I normally try. I was hoping I forgot something with my old age, but I guess not.
What's was strange was this machine was working fine for about a year, no problems. I've done normal Windows
Updates with it no problem. I've done multiple BIOS updates both with MS Windows Update and Dell Support Update, no problem. This last time, the latest Windows BIOS Update (Not through Dell Support Assist) happened
when I wasn't near the computer. I caught it trying to reboot and then it died with a no boot with the power
button flashing yellow/white 2/3 (if I remember right). Swapped DIMM's no change, Reseated DIMM's no change, Tried one DIMM no change but when I tried the last DIMM by itself, it booted.

That's when I tried taking out the CMOS battery, unplugging it and holding the power button 30 secs. Reinstalled both DIMM's and we're back to no boot. I even downgraded the BIOS to 1.16.1 and then to 1.11.1, still no boot. The only time it will boot is with the known one working DIMM in slot 1. HWINFO shows no DIMM in slot two yet has odd numbers in the label though.

This PC is on a UPS BTW, so I know there were no power issues. The only change I made to it when I got it was to change from RAID to SATA, other then that the BIOS is factory. I am running a Samsung 990 SSD in M2 Slot 0 and a 980 in Slot 1, no spinning disk.

Sounds like that module bit the dust.

If you're still under warranty, contact Dell tech support for possible RAM replacement. Have your Service Tag available when you contact them, but don't post it here.

It's really not that farfetched that a bios can affect memory.

Did you know Dell servers can 'repair' ECC memory, not just clear errors but actually apply voltage and fix the problem?

This used to only be possible by sending the memory in where special equipment would do this.

Now maybe our little XPS machines can't do that and possibly the memory just died even if you had only rebooted.

Yeah, In contact as we speak. Thanks for your help.

@Alan K.  - Any update on this? Is Tech Support going to replace that RAM module...?

Yes, tech support sent a replacement. In fact they sent two, a Samsung M323R4GA3BB0-CQKOD and a Hynix HMCG88MEBUA084N. My concern is that the two memory DIMM's don't match and may not go into Dual Channel mode or may have issues with it down the road. What I did was to replace the known bad one with the supplied Samsung M323R4GA3BB0-CQKOD and kept they old working Samsung M323R4GA3BB0-CQKOD in DIMM 2. Everything seems to be working now. I think I'll keep it that way unless there is some reason to replace both sticks with the two they sent. Opinions?

Yes, they already said they want the defective. Thanks for all your help.