That message indicates that your temp sensor is bad. It may be that the connections have a small amount of corrosion on them. The sensor mounts into the front header (plumbing side) of the heater. It has two small black wires attached. Look for these wire at your control board. With the power off, carefully pull the connectors off the board. (You won't get shocked, but a dangling wire can short a powered circuit board) Sometimes just putting them back on will solve the problem. If not, you need a new sensor.