Part cooling fan won’t work anymore A10

[Chaogeo]

I upgraded my hotend, everything prints okay, changed the marlin code for the new thermistor cartridge etc etc.
I have done something whilst ya king about the cables and can’t work out where the fault is.
The part cooling fan no longer works.
There is some voltage across the 2 pins on the extension board but the fan won’t spin up. Makes a funny noise like it wants to but doesn’t.
Pluggin it in to the main board on one of the empty fan slots it immediately works, so not the fan.
Have I buggered my extension board somehow? How would I check?

[Chaogeo]

I believe I have fried the mosfet that controls the part cooling fan. Anyone able to identify which component this is so I can check?
Gt2560v3 board, part cooling (not extruder Heatsink fan, then one that cools printed parts) mosfet component?