Dell Precision T7500 Not booting, lights 2 then 1-3-4

Have a problem with my Dell Precision T7500

Bought second hand and its working fine for months, today I switch it on to find a problem. Its no longer booting. I've swapped out all the parts I can and the fault still remains. I've unplugged everything unnecessary. I don't have a PSU or motherboard to swap out.

The diagnostic lights on the front give the following display:

Number 2 lights, split second later 2 is off and 1, 3 and 4 are lit. This then repeats a few times then the PC shuts itself off and tries to boot again.

Any thoughts on whether this is the PSU or the motherboard? 

Thanks in advance for your help.