Mac Pro - Early 2009 - 4,1 wont boot under OSX Having some issues with Mac Pro I’m hoping you can help with.
I received a Mac Pro 2009 4,1 from my work place after a user noted it ‘it just died’

Our guys here looked at it and could only conclude the fault could be hardware related but they were unsure what exactly. 

I took it home and installed a hard drive I had to hand with Windows 7 pre-installed, powered in on and after about a minute it loaded Windows normally despite not hearing the Mac start up tone. 

While browsing around in Windows I noticed the keyboard and mouse would go unresponsive intermittently. This was temporarily resolved by restarting Mac.
This gave me some hope so I installed El Capitan on an external drive using a friends Mac, installed it in my Mac. It fails to boot.

Symptoms:

Blank screen on power up

No start up tone is audible on boot up

The power led appears and is solid.

All fans spin up

The hard drive spins up despite not booting

The red led on the processor tray flashes briefly on power up – as it should

Keyboard num lock led doesn’t light up

What I’ve done so far:

Reset the PRAM battery

Replaced the PRAM battery

Swapped hard drive into every available bay

Swapped graphics card into another available expansion slot

Successfully booted to Windows from HDD

Tried booting from Ultimate Boot CD to run diagnostics. Fails. Probably has something to do with it being an ISO file?

Attempted firmware restoration here: https://support.apple.com/kb/dl976?locale=en_GB
This fails due to the power button remaining solid when held in

So at this point I’m fairly certain (unless I’m missing something) the fault is hardware related. Whether it’s the backplane or the motherboard tray that needs replacing I’m not sure but I thought I’d run it by the community for any suggestions before sinking money into either, or both. Any help is appreciated.
You have moved drives around, what about memory? Set it up with the minimum memory, then try with other memory modules, maybe you have a bad stick or two. Maybe one of the memory daughter cards is bad. Swap those, will a MacPro boot without an (empty) daughter card? Not sure about that

Also try unplugging the Optical drive.

You tried the Video card in another slot. How about another video card?

Anything that can be removed, should be removed to see if it is at fault.