The control system has detected less than 390 ohms from the Temperature Probe on top of the water heater.
Check all wiring between the Temperature Probe and the J7 Socket on the CCB (page 36). Repair/replace anything worn or damaged.
Unplug J7 plug from CCB, check resistance between the two black wires in the plug end with an ohm meter. If resistance is at or below 390 ohms – replace the Temperature Probe.
Call the technical support phone number shown on the water heater labeling for further assistance if the problem has not been corrected after performing the procedures outlined here.