After replacing the motherboard with an identical used one I was able to use both other memory slots too  There was a pin missing within the CPU socket of the old motherboard.
Hi, I've an used Optiplex 5040 SFF which was sold with 1x 8 GB SK Hynix HMT41GU6BFR8A (2Rx8 PC3L - 12800U - 11 - 13 - B1).As the unit supports up to 16 GB of RAM (4x 4 GB or 2x 8 GB) I've decided to buy another used pair similar to what I've already installed.So I got a HMT41GU6DFR8A (difference is only fifth character from the right - D instead of a B) which has the same specs printed on (2Rx8 PC3L - 12800U - 11 - 13 - B1).After reading some manuals and community posts in regard of dual channel both memory modules should be placed in slots with white levers marked with numbers 1 and 2. The slots are like this: 3 black \             channel A1 white /4 black \             channel B2 white / Unfortunately the system won't boot, not even the BIOS/UEFI - the power LED flashes amber and that's it. No beep code, nothing.After trying different slot populations I figured out the only working one would be 3 and 1 - and then only single channel.According to CPU-Z both modules run with exactly the same speed, timings, voltage. I've also removed the CPU and installed it again as well as cleaned the memory modules and slots - so it shouldn't be any contact issue. Next try was to get a supported 16 GB kit so I went over to the Crucial website, filled in Dell Optiplex 5040 and ordered a suggested pair of memory - unfortunately one of the two modules was DOA. Trying to mix memory also only works in slots 3 and 1. Question: why does populating anything different than 3 and 1 not work? I mean even if it would be only single channel at least it should work by using 1 and 2? Any suggestion regarding memory which definitely works in this computer? Thanks in advance.
Optiplex 5040 SFF DDR3L memory population question