MacPro 3.1 doesn't start, but starts with a "Hack".

Yesterday I wanted to book up the old MacPro 3.1 (Early 2008) The last time it ran was about a month ago. It only serves as a video watching system by now.

Nevertheless, it didn't start. No main fans moving. The Graphics card fan moved.After about 0.5-1.5 seconds it turned off. I tried several times always the same.

I had only one problem with it during a cleanup once in the past, where it didn't start.I think it was a loose graphics card at the time, so I opened and cleaned him.Wasn't that dirty to be honest.

Still not working, a few searches and the usual suspect is the Power Supply Unit (PSU). I removed it and a visual inspection suggests it's fine. Capacitors and Coils look fine. Cleaned it, put it back together still nothing.

While reading about trouble shooting, I noticed a red blinking light. CPU A OT and CPU B OT.I tried the button next to it and the SYS_RES button as well. At some point I did hear the startup sound, so I plugged the monitor back in and still the same behaviour as above.

However, after several trials I figured out I can press the power on button several times and hold it. After 3-4 times, the two LEDs CPU A OT and CPU B OT will light up permanently. Once that happens the next time the MacPro starts fully functional, but with the fans on full blast. I didn't find this behaviour documented in the manual, but might have missed it.

Mac Pro Service Manual

I downloaded the hardware diagnostic tool (Link) and put it into

/System/Library/CoreServices/.diagnostics and ran it.

No warnings. The extended test ended as well with "No trouble found" It took 1.5h, so I think the PSU is fine. I'm now running the test again with full RAM installed, out of desperation.

I don't know how to proceed now.I suspect it's some kind of error in the thermal sensors. They're plugged in correctly as far as I can tell. I didn't remove the CPU coolers as I don't have thermal paste. Is there a way to confirm one of the sensors are at fault. Also note they only are lid up after the "Hack", so maybe this is just a special mode?