D21 is the flyback diode for the FAN2 output. It's very unusual to see a flyback diode fail, assuming it was fitted the right way round in the first place. If you only connect ordinary brushless DC fans to the FAN2 output then D21 isn't needed anyway. The purpose of D21 is to protect the mosfet TR1 from the back emf if you connect an inductive load such as an air pump to the FAN2 output.
TR9 is the mosfet that switches the FAN1 output. I agree, it looks damaged. A short circuit across the FAN1 output would cause this kind of damage.
Which fan outputs are working?
From your signature, I presume you are in the UK. I can replace the TR9 mosfet, also I could replace D21 if the PCB pads for it are intact - although you probably don't need it, see above. Send me a PM if interested.
Edited 1 time(s). Last edit at 06/11/2018 06:31AM by dc42.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].