Error A903 means that the controller malfunctioned and the water heater was disabled.
To fix this issue, you have to replace the control board.
Zwell is dedicated to getting homes closer to net zero through the installation of new energy-efficient hybrid water heaters. We are not currently equipped to do service or repair visits but we want to be as helpful as possible. Feel free to reach out to us here or for more information, you can contact the Rheem Hybrid Technical Services Department at 800-995-0982.