Sorry 7040SFF

• Dirt
• Bent pins
• Faulty MB

MB's are quite reasonable on eBay.  Remember to include form factor/size in your search term (MT, SFF, etc.).

That's ok, no worry.  I see it's in your subject line now.

I'm not sure if this is telling of anything however looking at the BIOS and Intel ME hardware info DIMM2 and DIMM4 are in single channel mode.   That's with any combination of memory from the first post.

However, pulling two sticks from a Dell 3080 Tower (HMA81GU6CJR8N-XN 8G DDR4 3200 and TD48G26S819-VC 8G DDR4 2666) where they are running in dual channel mode on the 3080 they also appear as single channel on the Optiplex 7040SFF.

Does this have any meaningful info on why DIMM1 and DIMM3 are not working?  It seem I can't get DIMM2 and DIMM4 in dual channel.

Also, testing the 3080's memory in DIMM1 and DIMM3 results in the same 2/7 led error at boot.

I've been given the option to replace the 7040SFF buy the seller with the assumption this is a motherboard issue.  Any other recommendations before I start the process and ship this back to the seller?   Everything from above appears to checkout?  All the RAM should be compatible with the 7040SFF?

I'm not 100% sure if you mixed the RAM or not.  If speeds are mixed, and all the RAM works, it'll work at the speed of the slowest stick.  Doesn't mean it never works, but mixing RAM in Dell's isn't recommended.  Sometimes Dell's can get finicky and throw an error code if just the brands are mixed.  It especially won't work if voltages are mixed.

I was hoping another might chime in as I'm mostly unable to answer questions about dual and single channel.

I'm less concerned about dual/single channel and more concerned about DIMM1 and DIMM3 causing a 2/7 led error when working memory is in those slots.

Replacing the machine (in turn the board) and everything is working as expected.  All RAM slots are populated and are in dual channel mode.

Glad you got it working.  : )