Whenever I am driving a little over 40mph I get a blinking D light on and it goes away as soon as i turn ignition off. I have tried scanning code with OBD 2 scanner but no luck. Any clue what is going on ?

 

You cannot read transmission codes with a scan tool (unless it's a Honda brand one). What you can do is stick a paperclip in the OBD-2 port as shown, and the light itself will start blinking out codes. Write down the number of blinks and you will have a clue as to what the problem is. The problem is, you cannot really do anything about these problems other than changing the transmission fluid and changing the 3rd & 4th gear pressure switches. If those two things don't help, you can only keep driving the vehicle till the transmission starts failing (then replace it).

BTW: Don't do this paperclip-jumper backwards or you will instantly blow a fuse in the fusebox. Not a big deal (I did it), but it will startle you. Then you will just have to find the blown fuse and replace it.