G400s pc24

Is there any reason why PC24 would be HIGH on bootup?

I have a number of GPIO connected to relays with pulldown on each one and only the one in PC24 is active on power up until my programme starts. It is being driven and the same on 2 boards I am testing so not a wiring or short issue.

Can you control the pin properly afterwards? Have you tried TinyCLR and netmf?

Yes. It works fine on board boards.