2006 4x2.66 Mac Pro won't boot from any media

Strip the system and remove the drives. And list the hardware,GPU.

Look for obstructions. Why remove the Riser? And what was last changed? Power outage? other?

What OS and what last installed. Tried from backup clone?

Are you using the original grey install disks?

One disk will have Apple hardware test instructions printed on it. Try following that to see if it can boot to run a test. (Hold D at boot IIRC).

You can try downloading the AHT images & burning them to a CD if you know the model number - there is a list of AHT images at…

https://github.com/upekkha/AppleHardwareTest

You could also try target disk mode to see if the hardware can boot & allow the internal disk(s) to be read on another system (assuming you have the correct leads - Firewire or Thunderbolt are supported).

How to use and troubleshoot FireWire target disk mode

Target disk mode is a firmware feature so it may help isolate the OS's or disks as the issue.

Don't forget about single user mode & verbose mode - they may give you more items to look at, but can be very confusing to look at 🙂

Mac OS X: How to start up in single-user or verbose mode

User Tip: Mac Pro silver tower (2006-2012) Replacement Graphics cards

If you are running Lion (which is the only real supported OS now for this machine) there are a couple options.

I don't consider a $450 card to be an option! I would consider a number of GTX 5xx or 6xx.

There are forums, consider Netkas.org a good forum, for help using PC cards that offer you some selection.

MacVidCards on ebay and MacRumors forum is a pretty good source but 32-bit EFI on this machine is the major limit in OS and in graphic cards.

Grant: I would say your tip should have more 3rd party sources and is oriented to 2008 and later at this time, not 2006.

It just occurred to me that I had Apple replace a graphics card for this model for free. I reported a graphics issue to them at the store & they said it was a known fault & offered to replace the card with a NVIDIA 8800GT card. It was fine after that.

It was several years ago now (I'm fairly sure it was around 2010). If you had Applecare it could be worth contacting Apple or an Apple Store, sometimes they go out of their way to help, but I suspect they no longer stock replacement cards for the original issue.

8800's were sold and were also 'frequently' failing but can be 'fixed' by baking in an oven. Not a great choice and can cost $170 but also can find flashed PC G92's - likely on ebay macvidcards

7300GTs have been failing or two of its capacitors do. 8 yr old machines lack of parts - the OEM 2600XT was for 2008 3,1 and those were under warranty replacement but X1900 or 7300GT?

I'm afraid I don't recollect what the original card was, the 8800 was offered as a replacement when I said what OS it was running at the time, I think it was 10.6 that meant Apple gave me the 8800GT. Earlier OS's would have lead to a different replacement according to the genuis behind the bar.