T5500 not booting. Powers itself off

F12 , pick diagnostics and check what happens.

I did all that. I don't think there is a discreet GPU as there are two spare power leads in there. There was continuous beeping with no memory. I installed two different sets of memory in slots 1 to 3 and the system did the same thing regardless. Numbers 3 and 4 are on constantly for a few seconds and then power dies, much as before. Power supply or motherboard? The diagram of error codes seems to point to memory, but I thought I had eliminated that by swapping out ram?

Thanks Mazzinia. Unfortunately the power dies mostly before I see that option. I saw it once but before I could press the down key it powered off...

Disconnect everything from the MB except the 2 power plugs from the PSU (disconnect power from all peripherals such as HDD/SSD). Try booting and watch for any changes. If the system still fails to boot properly remove all RAM sticks but 1. Try again. You could also remove all RAM to make sure the MB indicates no RAM installed. Also if you have a discrete GPU then remove it and see if the system boots (I am not sure if your CPU has integrated GPU).

Also the diagnostic light chart for the T5500 is on this Dell support page.

did you try the small button behind the psu ? ( that should do a self test of the psu ) 

Also, how long ago did you change the cmos battery ? ( aka cr2032 coin cell battery ) They can cause a lot of weird quirks if ko

Hi,

try to remove the gpu and see if it keeps powered without it

Hi again. Yes I tried the button and that confirms power is okay. I also swapped out the CMOS battery. I'm lucky now if I see the bios start up screen. Interestingly the system seems to die but then 30 seconds later the fan briefly spins and it dies again!!

Hello again. I have used all different combinations of RAM that comply with a chart of the usable options mentioned in an earlier reply. I can't imagine that RAM is the issue. The same thing happens with the GPU removed unfortunately. The power supply passes its self test, but I feel that this is the problem somehow. I think I'm going to give up and buy another! They are not expensive on auction sites. Thank you for your kind help all of you. 

Well, at this point the choices left are limited, either the psu ( and the test is not efficient ) or the mb (some cap or element tied to the power rails, maybe )

You're welcome.  : )

The green light with the test button being pushed can be a bit deceiving.  It doesn't necessarily mean that the PSU is all good.  Sure, it has power, but even a bad PSU can spin the fans when the test button is pushed.

Good news is the green light means that your peripherals are good.  Otherwise, it would be unplug peripherals one by one until the green light comes on with test button.

It goes back to my mainly thinking that the PSU is toast.  Not too unusual in older models.

Thanks for all your help. I bought another old Dell from Ebay. I swapped the HD from my original machine, and to my dismay there was blue screen of death and then at reboot the offer to repair windows which failed. I dug out a cloned drive from a year ago which booted up fine, and then gave the failed drive one last go and it booted!!  Happy now...

I'm glad you got it working.  : )  I'd definitely want to keep stuff backed up in case that drive fails again.  I'm quite surprised things went that way.  Sometimes it just has to be something we didn't think of, especially when some PC antics point at the PSU.

How did you make out with your PSU replacement?

I had a similar issue and ended up replacing the PSU with a standard ATX one. It took a little bit of rework, which included ripping out all of the stock wiring, but I got it back up and running reliably.