I’ve reported a problem with my nas a few weeks ago (here) and my plan to get it back to productive work in a reliable way.

As part of this plan I did order a OEM replacement fan: searching an equivalent model with the same small connector proved to be a waste of time.
After a few weeks it finally arrived and yesterday I picked it up at the shop and replaced it in the NAS.

The old fan had some substantial play in the axial direction and at low-speed failed to spin up, for this reason I had high hopes with the replacement.
It was a quick job requiring only 15 minutes and then I was ready to use the NAS.

Mirror rebuild worked fine, compressing a file with the PC CPU with source and destination on the NAS worked fine.
Active disks were at 43C and the CPU at 48C during the rebuild; a few minutes after it was completed the disks were down to 35C and the CPU at 43C.
With 19C of ambient temperature this is not an extraordinary achievement, but are manageable temperatures.

I felt confident, even if not overly confident, that the fix was going to work as desired.
To be sure that this was really the case and that the overheating was not going to hit me unexpected in the future I did a heat torture test by compressing a 140GB file using the NAS CPU instead of the PC CPU.

This time unfortunately the NAS did behave as with the older fan: after some time it died on me.
The latest reported CPU temperature in the web interface was 54C and it survived long enough to put a warning in the web interface telling me that it could not compress the file.
The NAS couldn’t be reached anymore (no web interface, no ping, no Synology assistant) and couldn’t even be restarted using the power button.

Next step will be to mount a heat sink on the current nonremovable stock heatspreader.
I have already found a suitable heat sink, now the challenge is to find the needed adhesive thermal pad.
I will lose the ability to use the 4th disk slot, but it is acceptable because future space expansions will likely come via a DS414slim for additional redundancy and performance.
And I still have the LS220D that can maintain some of the lower value data.