1) All tankless water heaters require voltage to operate electronic sensors and circuit board.
This is true for gas and electric tankless.
2) Check that circuit breaker is not tripped. Push circuit breaker all the way one direction and then all the way back other direction.
3) There are fuses located inside the tankless unit, so check your manual for location of each fuse.
Manual is filled with useful maintenance and troubleshooting steps.
4) Before calling service technician, open following link for troubleshoot resources including manuals, Rheem on-line chat, and useful links for maintaining tankless water heater.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html5) If you had nearby lightning strike, I don't know if your tankless is protected from surge.
I recommend whole-house surge protector for tank and tankless water heater electronics,
and household circuits. Surge protection is installed in main breaker
box, and is replaced after each surge event. Separate protection needed
for co-ax TV satellite, co-ax surge protection can be found on some ordinary power strips.
http://waterheatertimer.org/Intermatic-trippers-and-parts.html#surge
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