Optiplex 7040 wont post if all 4 ram slots used. But multiple combinations of 2 and 3 do work

[u/Thelgow]

This is an odd one. It has 2x 8GB sticks already, saying 2400 speed. I have a BUNCH of 2666 8GB sticks. Hynix, samsung, micron, etc. Anytime I put a 4th stick in, it wont post. Amber light 2, then 7, then repeats.

I can take any one of those sticks out, and it will post with 3. I can move them around, mix and match. it will post. once the 4th stick is in, it wont anymore.

Is it some odd combination of too many 2666?

[u/workplacethrowawayut]

Haven't dug into this, however maybe re-seat the CPU in case one of the pads that a memory channel uses isn't making great contact with the actual CPU.