Planning to add extruder and stepper driver cooling fans that I want to control via Aux-2, outputs D40, D42, D44 etc. I am testing control of those outputs via pronterface and trying the commands:

M42 P40 255
M42 P40 0

etc, but so far am unable to control the state of those pins. I suspect that I need to tell the marlin firmware to configure those pins as outputs, or to allow control via M42 command. Ideas welcome.

your command is missing a letter

m42 p40 s255

you need the s

Yes, yes I do need the "s" thank you.