@silveroranges said in Duet 3 Heater failed open:
the heater is a 350mmx350mm 24v heater that is 430watts
That thing draws ~18 amps - did you have to replace the 15A fuse, and if yes: with what?
I think you’ve blown the MOSFet - take a look at your board for visible damage of a chip. Can you publish a good narrow shot of the board?