A1289 model. 
This is the heavy 2012 machine, single CPU, 4 core.
Sometimes boots then dies, other times dies during startup.
Eliminated most suspect items - swapped out memory, video, SSD drive and no change.
It could be something else like processor tray or backplane but can't swap these out so next check is swapout the PSU.
What do you reckon, and do you have a tested, guaranteed PSU?
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It makes me happy , that people are involved in such projects , I myself was crazy about it five years ago ...
I recommend doing a full system reset , and then we will get closer to the problem ...
My project was MacPro 1,1 update to MacPro 2.1 , I have parts from A1186... but it will not for you ...
The most ideal project is From MacPro4.1 to MacPro5.1 - it works better than the originals 5.1 ...

Resetting your Mac Pro Logic Board can resolve many of your Mac Pro’s system problems. When resetting the Logic Board there are
3 Mac power management reset switches that you can use to troubleshoot various symptoms:

System Management Controller Reset (SMC): Used to address issues related to power.
Real Time Clock Reset (RTM): Used to address booting issues.
System Reset: Switch used to determine if your Mac can’t consistently boot from a cold start resulting in power supply issues

Resetting the System Management Controller (SMC)

On newer Macs, such as the MacBook, Intel iMac and Mac Pro, the Power Management Unit (PMU) on the logic board is called the
System Management Controller, or SMC. Earlier models such as the Power Mac G5 and Power Mac G4 had the Power Management Unit.
The SMC is a chip on the logic board that controls all the power functions of your Mac and replaces the function of the PMU. It tells your
Mac when to turn off, turn on, wake, sleep and remain idle. It also handles system resets from various commands and controls the fans.
Resetting the SMC can resolve issues related to your Mac not starting up, not displaying video from your graphics card,
sleep/wake issues and fan noise.

Resetting the SMC does not reset the PRAM (parameter random-access memory) or resolve issues that have rendered your
Mac unresponsive; this is usually resolved by restarting your Mac. If your Mac unresponsive follow these steps in order until your issue is resolved:

1. Force Quit Applications (Option + Command + Escape).
2. Restart (Control + Command + Power Button).
3. Force Shut Down (Press the power button down for about 10 seconds). (should beep).

If your computer still shows signs of the Applications or Finder being unresponsive after you have restarted, then try resetting the SMC.
To reset the SMC on a Mac Pro:

1. From the Apple Menu, select Shut Down. (If your computer is not responding follow the above step 3 to force shut down.)
2. Unplug all cables from your Mac, including power cord and monitor cord.
3. Wait 2 minutes.
4. Plug power cord back in, making sure not to press the power button at the same time.
5. Reconnect the keyboard and mouse to your Mac.
6. Press the front Power Button and start up your Mac.

An alternate way to reset the SMC:

1. From the Apple Menu, select Shut Down. (If your computer is not responding follow the above step 3 to force shut down.)
2. Open and remove the side access panel of your Mac Pro.
3. Press the SMC_RST switch that is located slightly below and to the right of the row of diagnostic LEDs. Use a nonmetallic,
nonconductive object to reset the switch if your fingers are too big.
4. Replace the side access panel.
5. Press the power button to restart.

Resetting the Real Time Clock

The Real Time Clock located on the logic board controls the date and time functions of the Mac. If your Mac is experiencing a boot issue,
resetting the RTC might resolve the problem.

1. From the Apple Menu, select Shut Down. (If your computer is not responding follow the above step 3 to force shut down.)
2. Unplug the AC power cord.
3. Remove the side door from your Mac Pro and then remove the RTC backup battery for at least 20 seconds. You may need to
remove the PCI Express Card to access the battery.

System Reset

The Mac Pro has a system reset switch on the logic board that you can use to restart the system while powering on. The switch can be
used to determine if your Mac won’t consistently boot from a cold start has power supply issues. To reset the system:

1. Start up you Mac.
2. Using a nylon probe tool or a nonconductive and nonmetallic tool, press the system restart switch located on the upper right-hand
corner of the logic board.

If your Mac successfully reboots after you press the switch, try shutting it down and restarting it by pressing the front power button.
If the Mac Pro restarts when you press the system restart but not from pressing the front power button, then your power supply may need to be replaced.

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@Andree27  many thanks for that long and detailed response.  I am not sure how much of what you say applies to my 5,1 Mac, since it sounds like detail for an earlier Mac.  I will cross-check against the specs for it. Another check that I just did was to pop out the DVD tray and confirm that the fan beside the PSU is running.  [PSU overheating would have been a possible trigger]    The startup is quite erratic - sometimes it powers off before even getting fully booted, other times it boots and randomly dies instantly.  According to the guys on the Mac Facebook forum, the PSU is the next item in the chain to check and so that is why I am asking about the cost of a tested replacement.
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It's my flair, and maybe it doesn't apply to you -

(I allow the possibility that the system is in conflict , then it is not possible to identify the exact problem
I'm not sure about bios , because if you have a newer operating system than the one specified by the developer ,
then work on bios (BootROM upgrade)... After a restart, miracles can begin , because the bios don't understand
what's going on) ...

Relating to the spare part - Then Americans are still in the best position ...
Compatibility - 661-5449 Power Supply 980W for Mac Pro 2009/2010/2012
Search by those numbers... Eb ay The cheapest 90£ ...


The cheapest with delivery -
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There is a very good chance to repair your block(Power Supply) , In 99 percent of cases
capacitors inflate , solder with soldering iron , nothing more can go out of order ...

Warehouse in england -

The function of the capacitor is - if there is a problem, it goes out of order to protect another electrical chain , it bulging ...
It works as a fuse ...
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Great stuff.  I got the PSU out.  It is 614-054.  I can't see any obvious distorted capacitor tops.  I am reluctant to buy a replacement unit that is even older than the one that I am removing but I guess there is no option.  Thanks!
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I took a chance and ordered one off Ebay.  It should arrive on Thursday.  I used to be an electrical engineer but I draw the line at dismantling that PSU.  Thanks again!
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It is suspicious that the capacitors are not damaged , but let's say the problem is with the power supply ...
You write that memory and graphics have changed ... Pay attention to it do you know what version of the
operating system was previously installed , If not, then you have to follow the standard configuration ...
Why is it - because , it never works with a more powerful video card , until the system is started and
and the above steps have been performed - the system is ready to install a newer system ...
(ATI Radeon HD 5770 with 1GB GDDR5)
As for memory - How many sockets do you are ? Suspicious behavior with electricity ...
I think there may be a problem here as well , you need to know the maximum amount of memory for
your model , and what is the maximum amount for one slot ...if you used a memory card larger than 8 gb ,
nothing will happen , I recommend leaving one card while the system is not running ...
Because I think it is not original and therefore conflicts occur , that the system is working
you put as many as you want ...

Dual-processor systems
2.4GHz, 2.66GHz, and 3.06GHz systems: 1333MHz DDR3 ECC SDRAM
Eight memory slots (four per processor) supporting up to 64GB of main memory using 1GB, 2GB, 4GB,
or 8GB DIMMs
Single-processor systems
3.2GHz system: 1066MHz DDR3 ECC SDRAM
3.33GHz system: 1333MHz DDR3 ECC SDRAM
Four memory slots supporting up to 32GB of main memory using 1GB, 2GB, 4GB, or 8GB DIMMs

If the processor is standard, it could be your system had a maximum el capitan ...
I recommend experimenting with memory , and pay attention to the graphics card so that it does not exceed
the standard ,( modernization will require more higher graphics card ) ...
My experience showed similar symptoms - the system launched a formal update , but nothing restarted after restarting...
I also think it's dead , but it turns out that the hardware is not ready for it ... Believe me, the symptoms were similar ...
I just ignored the standard requirements , because i think it is the same as windows ...
After two days of searching the internet, the system was fine ... (Mac Pro 1.1 up (2.1) with el capitan)

Maybe it doesn't apply to you , I wish success , give a message how's it going !
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Great advice!  Memory was upped from 6GB (3*2GB)  to 32GB(2*16GB) a few weeks ago and it worked fine.  After that I added an SSD and it worked great.    Yesterday I replaced the 32GB with the original 6GB and the problem was still there.  I removed the SSD and the problem was still there.    No system updates made recently and it is running High Sierra.  Video card removed and it booted up but died after a short while, so I think it is not the source of the problem.  Replacement PSU on order from EBay so will try that when it arrives.  A circuit diagram would be nice, in order to identify all of the possible items that could cause the machine to halt.  I would have hoped that things like temperature sensors showing overheating would give some kind of visual feedback.  If the PSU does not fix it then I guess the processor, processor tray or backplane could be suspect.  Thanks again! /Louis
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PS I did not see all of the capacitors in the PSU and so I do not know for sure if they were ok.   
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That's a good thread - relating to the processor , i have had a sad affair with the window system
the motherboard mount was broken and the processor radiator was tilted , when you start the system,
it simply shuts down as if there is no power , I suspect you already know PSU , until I notice that
the processor radiator is tilted ... I think the ap ple system is similar... And it could be
protection works , sensors works , prevent system startup ...
What you need to know about the processor - Thermal paste , Thermal Paste Grease Compound ,
It should be flexible, not rigid (hard), because then cooling cannot take place ... There must be good contact
with the radiator... If that doesn't help, then the problem is in the technical direction , components on a
tray...It happens that the processor is burned, but rarely ...if necessary processor tray , the minimum
price is 160£ ... but strictly for your model , 4.1 and 5.1 are separate but visually identical ...
Thermal Paste Grease Compound recommend halnziye HY710 silver , it never stays hard ,
other does not work for long... It would be nice if you had friends in America ...

PS .
Possible - bad contact with the radiator or thermal paste is like wood ...
You might be interested
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PSU should arrive tomorrow. Before I dismantled it,  I got temperature readings from the Mac one time when it was running.  None of them were too high so now just wait for PSU. Thanks
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PSU arrived.  I thought it wise to just do a final check with my original PSU before installation of the replacement unit.

I replaced the original PSU but did not screw it in.

To my surprise the machine started and stayed alive with the old PSU.  It has stayed alive for over 10 hours now with no problem.
I'll keep the replacemetnt PSU on standby and test it at some point but for now the Mac seems fine.  

I think it is too early to be sure that the problem has gone but it may be there was some oxidation on the PSU connector contacts.

Thanks again for your advice! 
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What you need to know about the power supply - if it has electricity but the system does not want to run
check power supply components , can be repaired, if there is no electricity, the winding coil is damaged,
new power supply ... Of course systems settings and configuration ...
What you need to know about the processor - Thermal Paste Grease Compound ,fresh and gentle (soft)
so that the processor always cools down ...
What you need to know about ssd - the ssd hdd must not fall asleep , there are problems waking up ...
Read this ...

I'm glad for you , it was a pity that there were expenses , I wish success !
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