[mrthinlysliced]

So you say the massive blower next to the ODY sucking air from inside the chassis blowing it out on the back is ok and the fan between
the SCSI-backplane and the pci-x compartment is also running, then I assume you are right that the fan not working must be the one
underneath the V12 case. It would also make sense that the ODY is kind of disabled, probably in the PROM (console=d), without the
required cooling.

Yep, massive blower, massively blowing .-)

Your comment sounds reasonable about why the ODY is disabled. Time to unmount the GFX bits and take some photos tomorrow.

Is that ODY fan attached to the gfx board or the mainboard?

The disappearing act of the V12 can be as simple as it's Dallas I2C chip gone wrong -- they have a reputation for it. The I2C bus is handled by the L1 and -- guess what -- monitors the fans also.

Aha. With the info from you both it starts to make a little sense. A little deconstruction for tomorrow, maybe try a boot without the ODY attached to see what the PROM says and if the machine works headless while I work out a plan of repair.