I have the same computer and it turns on automatically provided the battery is still good. I usually turn it on myself to check that the gas is set correctly when gearing up to match what I have in the rental or my own tanks.
Thanks! It's great to know it. Well, I forgot to mention that I always use air, no nitrox. So all I do with the computer is turn it on at the dive center to check the battery is still OK prior to the dive (so as to have time to replace the battery if it's low), and then I turn it off because it can take a long time while I wait for everybody to get ready, get to the boat and get wherever we're diving, and in that way the battery will last longer. I usually turn it on again when we reach the dive site, and although I've never forgotten to do so, I wondered what would happen if I did.
Why do you want to do that?
If you do not trust it then get rid of it.
You are over thinking the issue.
Get another one(different brand) as back up.
I do trust my computer. What I do not trust is my memory. And I cannot get another one, I'm afraid.