Tip - Mac Pro fails to boot up, no chime.

Have you checked the battery? I seem to recall they generally last 5-7 years on average.

My rear fan at the back of the computer is not turning at present and I think they are only used when required according to the internal temperature.

I think you will find that when operating properly, all the fans spin slowly, even when nominally "not needed'.

This could be an indication of impending trouble with that fan, but if it is able to spin faster when set faster, that may be OK for now.

if the power-on light is blinking a pattern, that often says, "not enough WORKING RAM to start up".

The corollary to that is:

If you remove all the RAM, and the processor is starting and the ROM is working, you can INDUCE the blinking power light error. If not, your processor may not be running.

I've tried leaving Mac off for days. Didn't work.

I've just spent all day following LED diagnosis.

Remove:

Hard drives • Optical drives • Processor tray and processor board (containing processors, processor heatsinks, and memory) • PCIe cards • AirPort card • Bluetooth card • Battery • PCIe fan • Processor cage (including fans) • Front panel board.

DIAG LED behaviour normal.

Whilst reconnecting the above one by one all behaviour normal after pressing DIAG button.

I am tempted to replace processor, but spending money and finding out I was wrong is stupid.

I don't think it would work if you remove the battery completely. You would need to replace it with one that was known to work, but this is just one of several areas to try. Also I think you need a minimum amount of Ram installed. Also I would remove the display if you haven't done so already.

How do you know if the processor is working?

Not something I've had to do but if you Google that exact question there is quite a bit of information.

I'll do that Ashley.

One other thing I forgot to mention was whilst pulling the power plug and plugging it back in during testing the Mac powered up without me having to press the power button on the front panel. Is this a clue as the what the problem might be?

How do you know if the processor is working?

1. Power off

2. Remove ALL the RAM

3. Power up normally

if the power light blinks a pattern of three, your processor is working. This is because those blinking patterns are generated in software.

Not sure about that business with all the starting up as soon as you attached the power cable. At the moment all sorts of crazy stuff could be going on.

One of the first things I did when I had problems was to install a different start up drive, which was a clean install of El Capitan with nothing else and remove all other drives. That reduced the risk of software problems and also allowed me to attempt dozens of restarts without risking damage to my main drive.

Have you tried starting from an external drive?

One of the simpler things you can do its to Alt/Option Boot. This draws a gray screen, even if you have no bootable drives. Then within three minutes shows an Icon for each potentially-bootable Volume. This code is all in ROM and it demands very little from your Drives to show an Icon.

Grant = thanks for that, I'll give the removal of the RAM a try and I'll try Alt/Option. So if the power light doesn't flash does this mean a processor issue?

Ashley - thanks, I have searched for installing ElCapitan on external HD. If I were to follow this procedure http://www.macworld.co.uk/how-to/mac-software/run-el-capitan-on-hard-drive-yosem ite-on-mac-3533305/ on my iMac would it then transfer bootable OS on ext hd for my iMac, or could I simply unplug and plug into my Mac Pro?

In my case I had a SATA on standby for emergencies of this kind and was simply able to slot it in place then remove the others.

You ought to be able to download OS X (any version is fine) on your iMac and simply set an external drive as the install location. You may even be able to do this on a USB stick. Then start while pressing the option key to hopefully select a drive connected to your Mac Pro. This won't be something you can do in 2 minutes but at least it could tell you if your drive is the problem. http://www.idownloadblog.com/2015/09/14/how-to-start-up-mac-from-bootable-media/

Something like this may work by using your iMac in target mode. http://apple.stackexchange.com/questions/153831/booting-an-imac-using-a-macbook- pros-internal-disk

At the moment we just want to hear that chime on the Mac Pro. In my case I couldn't reset the PRAM until the video card was removed. After the card was gone I reset the PRAM first time and it was all fairly easy apart from the cost of a new card.

Removed all RAM and inserted power cord. Again the Mac booted up without me having to press front power button. power light remains constant, no flashing.

Could it be a faulty front power switch panel? or is it the CPU board?

Hard to know for sure, but it might just be something related to that power button. I seem to recall there used to be a hardware diagnostic test that could be run, either by inserting some CDs that came with the computer or pressing the D key on the keyboard at startup. Have you been able to attempt a startup from an external drive?