OK, think I have worked this one out. I used the dynon recommended Lycoming 6 set-up but it looks like the pitot is not configured correctly. Found the page in the manual that refers.
We actually labeled it that way with intent, to reflect the way they do it in certified land (caution when off, no caution when on). But you're welcome to change it to suit your preference.
I understand that but mine is sensing ON/OFF the wrong way round! With power OFF it indicates on, with power ON it indicates OFF with a caution. I can't see how I could have connected it incorrectly as there is only one wire to connect. I'll look at the sensor set-up at the weekend.