I had a "thermal event", restarted computer, no problem, came back a few minutes later, computer won't boot. I can bypass the power on/off toggle switch and boot just fine, but this bypasses the "thermal event" shut-down protection. Is the prob my MB?

Forget everything about overheating vents being blocked, replacing CPU Fans, etc. It is the Motherboard that needs replacing, nothing else!
Dell are very much aware of this problem; it is a design fault caused by using the cheapest parts they could find. Particularly in GX270's, a particularly cheap and nasty bit of kit!

Rather than spend millions re-calling all the PC's, they decided to do nothing! Instead they waited in hope until the PC's went wrong and that they were out of their warranty period!

Yes, I understand that it's overheating, but why won't the computer boot now? I haven't burned out the CPU. And overheating takes time to occur before shutting down the system. I can't get things to start normally, but I can bypass the start button and get things to start (so the CPU isn't fried). That's why I thought maybe the motherboard had been damaged at the start button connector, but I can't be sure. I don't want to replace the MB if there's another fix. Thanks.

Is this still running? I don't think it's a bad MB if it's running.

No petrusromanus, it IS the mainboard and Dell has confirmed this with me on the phone not 3 minutes ago. They will replace the mainboard even though it may be out of warranty.

The issue of overheating with GX270's is not related to the amount of heat that is being put out, but the quality of the capacitors that were used on the motherboard.

If you look at the capacators on the USB/Par/COM port side of the processor, you'll probably see that they are marked as "X". X means bad. Very bad.

The problem is that the capacitors have failed and the thermal circuitry no longer works correctly. As a failsafe, if the thermal circuit isn't working, the computer automatically shuts itself down to prevent damage.

You need to have the MB replaced. No ifs, ands or buts. Dell will honour replacements even off warrenty.

You probably will have to switch it yourself, but it's not too complicated.

ALSO: Make sure you get a replacement power supply as well. The blown capacitors have some sort of weird voodoo PSU aging effect that causes it to fail in the near future. Replace both at the same time.

Hopefully this works for ya. :D

Its the processor not the MB. Sounds like your system is over heating.

A thermal event is the common error message when the Dell motherboard has died, particularly on GX270's and GX280's. If you open your case, carefully take a look at the capacitors, without touching them, to see if they're bulging or leaking. This was the common Dell motherboard visual confirmation. In some cases, Dell has extended the GX270 warranty for motherboards, by the way. Its worth checking, anyway. (I'm a Dell certified pc tech by trade.)

Ive had users call up for issues like this...9/10 times the motherboard it fried. Open the case, check the motherboard for any visable signs. ie melting, blackend, poped capacitors, and the like. If you see anything like that you should just get a new motherboard. Your might also end up replaceing some of the other hardware as well, because some of the other hardware could of been damaged by either the heat or what ever caused the motherboard to go bad.

It could be system overheating. Or worse popped or burned off capacitors. Check http://www.theunixtips.com/hardware-dell-gx620-sff-htpc-bootup-problem-burned-up-capactiors/ which discusses couple of things that can be done.