I got a am15, noticed the heaters would emergize with the tank empty. I would disconect the heaters while troulbshooting.

Found that even with the float/temp probe unhooked the contactor would still pull in, more odd though is that the contactor would pull in with the unit off and only the main disconect on.

Could the relay board be bad, on the diagram it shows the contactor gets energized though the K9 relay, the relay is fixed on the board so I cant test the output.