@Marius-Breuer note further on in the Noctua white paper - "External pull-up is not necessary as the signal is pulled up to 3,3V/5V inside the fan."

However, Noctua also deprecates using a single transistor and relying on the pullup and favours a two transistor drive circuit for the PWM (also further on in the white paper). I have found with some fans a need to add an additional external pullup, and actually I've generally pulled up to 12V (though not with a Noctua fan, and I note they say don't pull to 12V) see e.g. https://forum.duet3d.com/post/306593.

@kooper swap the cables between the boards to rule out its an issue with those maybe.

@fcwilt Thank you. That worked. But I almost got a heart attack. I Checked the connection, checked it again, then checked the connection. I flipped the switch and I heard a strange sound! It was just my arm had snagged the curtin and it popped on the rod.

Anyway, it worked. Thank you