Don't trash until you check the power leads. Upon installation of my most recent autohelm it passed the "at the dock" trials but after leaving the harbor for the sea calibration it did just as you describe, tiller hard over. Reading the instructions I found a very small print note that when this happens the leads to the tiller control unit should be reversed--did that, works fine. Dunno why it would work at first then stop working, though; but thot you might check it before giving up.