Not sure if this is how it supposed to work, a bug or Pilot error .

 

I have set the required altitude, taken off and have armed Nav on the AP but not engaged AP. I  trim to the vy speed and manually fly to intercept the GPS. Whilst still climbing I engage AP to capture Nav mode but all hell breaks loose and the aircraft nosedives, trimming all the way down. On the flip side if I don’t engage AP until I reach altitude then everything is OK. 
 

it may be that’s it’s all or nothing on the AP front but that’s not what I’m used to. Could someone enlighten me? FYI this is in Cessna 172 but it’s the same across the board.

I would recommend always engaging the AP first, then NAV mode.  That works for me and is standard practice I was taught.  Since that's the way I've always done it I haven't noticed the behavior you describe, which sounds like a bug.

Thanks for responding. I’ll give it go.

Thank you - that worked. 

we've discussed this, your're AP is conflicting with other settings in devices.

I am having a problem with the NAV key command. When ever I press it  am getting LVL instead of GPS.