Ender 3 fan not running - seeking diagnosis advice please

[AutoWiz]

Assign it to a different pin on the mainboard. Depending on what mainboard you have that is. If the mainboard has an unused heater like for E1 then you can re assign that to be your fan pin, for example. But just to be sure it is your mainboard how and where are you testing for power? The fan should be unplugged from the mainboard and you should be testing the pins on the board. I say this only because if the power wire touches ground it will be pulled down to 0v and if the wire is broken and you are testing for power at the extruder then you will not see the power at the mainboard and both are potential causes as them wires move about as the printer runs.