My dolphin will not go into reverse mode

by Paul B
(Linn, MO)

Hi y'all my dolphin will not go into reverse to back away from the walls. I've narrowed it down to the board and the only thing that does not give a reading is d14 diode that has m4 on it. I normally use NTE for all my parts but we've come up empty on results. Can anyone help me on the specs of it please?