Have you checked the BMC SEL log to see what the system is reporting as the problem/
is it possible to read the log file from shell or do I need to boot into an OS
You had me wondering on this one so had to go try it.
I disconnected all the fans and back-plane on a Dual CPU SR1625UR so that only the motherboard (& RMM3) is connected.
Powered on just fine and completed POST (then I shut it down before I over heated something).
The only time I have experience an issue were a fan load was required to POST end up being the power supply.
Power supplies have a well published maximum current draw.
What is not so well published is that many power supplies also have a minimum current draw per PS rail with separate rails for CPU0 & CPU1.
If your issue is straight PS under loading, check the voltage between pins 4 (gnd) & 8 (12v_CPU1) (2 pins closest to the 24 pin power supply connector) on the CPU power connector (J3K1). Should come-up 12v. You could even attach your load at this point to confirm that the fan load is PS related rather than Mother Board related.
Are you running single or dual CPU packages?
This 12v rail powers the 2nd CPU also (numbering is CPU0 & CPU1), so if you don't have dual CPU's and no fans, the draw on this rail is very tiny.
So No load on the PS rail = No power out of the power supply for that rail = No power good signal from the power supply to the mother board = No POST
Personally, I would use the mother board fan headers and let the mother board control its own fans.