Mac Pro 2008 Refuses to Boot My Mac Pro 3, 1 began rebooting randomly one day and the next it wouldn't come back on. Pushing the power button results in a fan clicking in to life for half a second and then all power is lost. The RAM board LEDs and Diagnostic LEDs OT A and OT B both flash momentarily. When the diagnostic button is pressed on the logic board the Trickle Power LED lights orange.

All my troubleshooting led me to believe the PSU was borked. But when on the phone to Apple support, around a week later, I tried one last time to turn it on and it roared in to life. It stayed on for a few days and I backed everything up. It hadn't been off at all, I updated from 10.10.1 to 10.10.2 and rebooted. No issue. I shut it down.

At 8am the next day I woke up to the computer trying to start, and I remembered that before the initial problems I had set it to wake at that time. But then it clicked off and it's been refusing to start ever since.

Apple support (over the phone) say they don't think it's the PSU as it wouldn't have started working again. What could it be? Could it be the automatic wake?

I have tried removing the memory boards and every other component one by one without any luck. 

Notes:

I'm not getting to any screen, grey screen, unhappy mac face etc.

Also, this, from the service manual was why I decided it was a PSU issue in the first place...

"If both overtemp (OT) LEDs come on immediately when the computer is turned on, a faulty
power supply could be one cause of this behavior. Replace power supply."
I ended up solving this. I had installed a SSD, and connected it to the hidden sata port. It worked for a couple of months. Then one day I had this issue. I took the ssd out and installed OSX on the first drive bay and it booted! I'm guessing the fan chassis must have been snug against the connector or otherwise some dust was causing grief. In any case. I'm glad I didn't pay an exorbitant fee for the rare aftermarket PSUs.