We'll need a lot more information that that, especially why you are asking this here.
When I turn off computer the fan for the psu continues to spin indefinitely it's quiet only when I flick switch on the back does it stop
Because I'm running out of options as to why the psu continues to spin and maybe it's the motherboard
Seriously? What part of we need more information didn't you understand?
What mobo? What CPU? What PSU? Give part numbers, not descriptions!
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