Just to clarify, I have tried using the M307 F parameter. I don't have a good way of measuring frequency but it doesn't seem to have an impact after the heater has been disabled.

Is this something that would require a change to the firmware?

Hi everyone,

Is there a way to change the PWM frequency of the heater pins (0-7) when they have been disabled for use as I/O pins?

Currently I'm following the instructions at:
https://duet3d.dozuki.com/Wiki/Using_servos_and_controlling_unused_IO_pins

I am freeing up the heaters for use as I/O pins by disabling them using:
M307 H# A-1 C-1 D-1

Then I can then control them using M42. There is an option for frequency in M307 so I'm guessing that the hardware is capable of it, I just can't find an option in the GCode.

Thanks in advance