An E5 error code indicates that there is an issue with your fan or venting.
If your fan is coming on, but then giving you an E5, you most likely have a blockage in your venting. This mostly pertains to the 40HI as the 40H models vent directly out of the front of the heater. Check inside of your vent pipe to make sure there is no debris blocking the flow of air.
If your fan is not coming on, then you have an issue with voltage or there is something that is not allowing the fan to spin freely. A common solution for this is to first unplug the water heater from its electrical supply. Next, remove the cover by unscrewing the four screws that hold it in place. The indoor mounted 40HI models have a ribbon cable connecting the display to the computer board. Please be careful as to not damage this wire. You will need to disconnect it using the black connector located 6 inches from the display itself.
Inside the water heater, at the bottom left, you will see the fan. Again, with the water heater unplugged, reach in and spin the inside of the fan to make sure that it spins freely. After doing so, on the right side of the water heater, you will see a grey box which contains the computer board located on the right side. If you remove the two screws on the face of the computer board, it will slide out freely from the box. Next, disconnect and reconnect all of the wires which are connected to the computer board to verify that everything is seated properly. Lastly, you will see two blue rubber boots on the computer board. Remove the rubber boot from the fuse closest to the middle of the board and check to make sure it is still intact. If it is not intact (the small metal wire in the center of it is broken), replace the fuse. Then, reinstall the board, replace the cover, plug up the water heater to its power supply, and see if your issue is resolved.