Optiplex 7040SFF DIMM1 and DIMM3 memory failure

Testing with known good RAM from DIMM2 and DIMM4 (M378A1G43DB0 CPB and M378A1K43BB1 CPB) in DIMM1 and DIMM3 results in 2 amber short pause and 7 amber then repeats 2/7 etc...  From Dell's 7040 PDF this appears to be memory failure.  Tested all sticks (above models and two MSL4U213FF8GX2) with DIMM2 and I don't get any failures.   Also tried to reseat the CPU same issue.

What could be wrong with DIMM1 and DIMM3 such that it always fails?

Dell Optiplex 7040SFF, BIOS 1.22.0, 4304.16 UEFI ROM.