CPU 2 Fan Failure on Precision t7910

That's what I expected too! But no I actually see CPU 2 fan failure. (This is a two-processor board, but still I would have expected it to be called 1.) On the motherboard though the fan connector does say CPU 2 fan.

Instead of taking off both heat sinks I'm just going to take off the one and swap out a replacement fan. One thing I learned since my post is that the fan comes out of the heatsink, so a new heatsink is not needed, just a new fan (and it seems like a standard 80mm one).

Is it always a good idea to reapply thermal paste when taking a heatsink off and putting it back on?

Ummm.  Dell used 0 based and not 1 based indexing.  So you should not see "CPU 2 fan failure" but "CPU 0 fan failure" or "CPU 1 fan failure".  There should be an "Alert!" in the message as well.  I would reseat both fan power connectors and try to boot.  If you still got the error, it might be worth swapping the two coolers (using new paste).  Not too difficult.  If you still get the same error, its on the mobo.  If you now get the other CPU indicating a fan error, well, you can get another cheap on ebay.  

Interestingly enough, Dell used 1 based indexing for hard drives.  So if the hdd fan fails you get "Alert! Hard Drive fanX failure"  where X is 1, 2, or 3.  

My understanding is that yes, it's a good idea to use fresh paste.  Also, it dries out apparently, so period cleaning off the old stuff and reapplying the new stuff is recommended.  Doing that really helped the temps on a Lenovo laptop I had.  This is every few years I think, not every few months.   But to test things if you have a replacment fan just unplug the old one in place, plug the new one in and see if it boots.  If it boots w/o error then turn it off (quick) and replace the whole unit (with paste)!    

I see that I didn't answer your question about using different fan /coolers.  There are two standard ones from dell.  search on ebay for "dell t7910 cpu cooler"  and you'll see that the OEM stuff is available.  Search for "dell t7910 cpu liquid cooler" and you'll see that they made a slightly higher performance one.  But with the latter, you need a different side cover for the case.  It's vented.  Frankly, I think that in may not be much better, but venting out the sides of the case (instead of one cooler venting into the other) makes for higher performance.   Standard coolers are about 50 bucks, and the "liquid" ones are 150.  Plus about 150 for the cover. I have standard coolers for my dual E5-2699A setup.

Thanks, the heatsink and fan actually come apart it turns out. The fan appears to be a standard 80mm 3 pin. Since the heatsink is just a chunk of metal doesn't seem like I need to buy the whole thing, just a replacement fan that I can pop in.

So you diagnosed it to a specific fan.  Cool.  Good luck.  Still trying to figure out why your unit uses CPU 1 and 2 and the manual says 0 and 1.  But consistency is the hobgoblin of little minds like mine I guess.

we need answers...