Ram error fixed when rotating module slot?

[Burrwonkey]

Hi, I have 2x8GB 4000MHz modules, let's call them module1 and module2.
They are in dual channel with module1 in slot A2 and module2 in B2 (2nd and 4th slot).
This configuration gives me 10+ errors on pass1 test 6.

Now, if I rotate the modules so I get module2 in slot A2 and module1 in B2 the errors goes away and I can finish pass1 without errors.

What can I learn from this? Why is this happening?

my setup:
memtest86 v7.5
i5 8600k on msi z370 tomahawk
galax hof 4000MHz XMP DDR4 1.4V
VCCIO at 1.2V
VCCSA at 1.2V
Ram works seemingly fine and boots up in Windows 10 and games are without instabilities.

[David (PassMark)]

If you swap them back to their original positions, does the error return? If it doesn't maybe the problem was just a bad electrical connection in the slot (e.g. dirt, dust or corrosion).

If the error does return then maybe one module is marginal (right on the edge of working / not working) and the conditions (timing and voltages) are slightly different in one of the slots.
Another possibility is that the bad address as being hidden in the second error free setup and aren't being tested. i.e. the bad addresses happen to land in memory assigned to the video card or some other hardware. Typical PCs reserve around 5% of their RAM for various hardware and data structures. This 5% can't be tested.

[Burrwonkey]

If you swap them back to their original positions, does the error return? If it doesn't maybe the problem was just a bad electrical connection in the slot (e.g. dirt, dust or corrosion).

If the error does return then maybe one module is marginal (right on the edge of working / not working) and the conditions (timing and voltages) are slightly different in one of the slots.
Another possibility is that the bad address as being hidden in the second error free setup and aren't being tested. i.e. the bad addresses happen to land in memory assigned to the video card or some other hardware. Typical PCs reserve around 5% of their RAM for various hardware and data structures. This 5% can't be tested.

Thanks for the reply! The errors return when I swap them back to their original positions. I can try to run a pass with just one module and test both slots to see if perhaps one of them is marginal tonight. Is one pass sufficient testing or should I let it run longer?

[David (PassMark)]

If you were always getting errors on the first pass, then doing one pass is enough.