CPU Fan not spinning on iMac 27" 2011

Hello,

I have a CPU fan on iMac 2011 27" that is not working. I have tested the fan on optical header and works, tested on another working iMac and works, used a different fan and doesn't work, have tested another working power supply but still doesn't turn.

I'm wondering if it's a motherboard / logic board fault causing this issue.

I have replaced the thermal paste on heatsink but only think I noticed is the safety ring that goes around the cpu heatsink screw to keep it in place is a bit fiddly but have still managed to screw the 4 screws all evenly to the logic board and the computer powers up and loads into system but mac fan control shows 0 RPM and doesn't spin.

The only thing I could possibly think is the heat sensor on the heatsink but after carefully looking at it, it seems to be all fine. Not sure if faulty cpu heat sensor would cause it to not spin.

I'm steering more towards the cpu head fan header (socket) that you plug fan connector could have some issue but doesn't appear to be loose at all.

Has anyone come across this fault before and could point me in the right direction, much appreciated.