Hello,
I have a honda accord 2006 model.The "D" light just started blinking minutes after a drain and refill of automatic transmission fluid from dextron III to Atf DW-1.What can the problem be?

Check for codes.

Make sure you are not highway driving with the D engaged. Push the button on the shifter stalk and then re-engage to see if it changes. It may be warning you of over revving. My 05 D does not blink but I have never used it at highway speed.