Thermal event alert on a Dell Optiplex

It could be a power supply issue or a faulty thermal sensor, or something else over heating.

Did you try the Dell Diagnostisc ?

I hope this helps !

Couple months ago I had 2 Optiplex's with the same issue.  They both needed new motherboards.  Suggest booting from a diagnostics cd (www.ultimatebootcd.com has a good one) and run some mb tests.  One of mine failed mb test almost immediately, other ran for couple of hours before throwing out errors.  Pretty good idea to try Dell Diagnostics, although in my case went through all steps and came down to the good old  Call your Dell rep.

When you replaced the fan, Did you also remove the heatsink from the processor, clean it's and the processor thoroughly, and apply a very small amount of thermal transfer paste? If not, do that.

did you put the thermal paste as suggested by dell. i had a similar issue with an optiplex gx60 i didnt use enough paste then the cpu will shutdown if it got to hot....i would notice that it will only happen if i let the pc on for 1 or 2

I have seen bad capacitors cause silimilar issues and many dell desktops check the caps on the baord to see if any are leaking or bulging (www.badcaps.net).

it can be caused by nearly anything :
-thermal : check the cpu temp in the bios, or use speedfan; if too high, apply heat paste as suggested.
-power supply : check if it can handle the load here : http://extreme.outervision.com/psucalculator.jsp
    or try with another one
-other : swap devices or start with the minimum (1disk, no cd, no extra add-on cards ...)      

I've seen 2 dell Optiplex gx270 pcs have this exact same fault - both had bad capacitors as per the links above - the tops were domed over - easier to tell by feel than touch....

I'll reiterate - you've got a bad motherboard (or bad capacitors in the mb if you prefer).  You can physically inspect the capacitors in the mb as stated above and look for any that are obviously melted, burnt, slumped, etc.  You can also run mb diagnostics (see my link above).  You've already replaced the processor fan and the power supply, so rule those out - just a thought - the power supply you put in has the same specs as the power supply you took out, right?  It's a possibility the thermal past could be an issue as stated above, but doubtful.  Run the mb diagnostics and see what errors you get.

you should check the caps that somerthing that we overlooked at look  at the caps around the cpu and if those are marked with a x you need to take a closer look to make sure they do not look bad. if you have a K or a T then that could not be the issue. dell had to replaced 3 of my mobos due to bad caps this is a known issue on GX270