Before calling service technician, Open following link for troubleshoot resources:http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
C2 11 is no ignition.
Do-it-yourself solution can include: clean burner parts and combustion chamber, clean air intake, realign igniter and sand carbon off igniter.
Other causes can be combination of factors.
Factors including improper grounding, incorrect outlet polarity, bad igniter, bad gas solenoid, low gas pressure, wrong gas, gas valve partially shut off, air in gas line, bad venting, loose wiring, bad sensor, bad PC-board, etc.
The product manual recommends yearly service.
Yearly service should include checking/testing each item mentioned above, plus checking that temperature is not set too high for volume of water used, checking for water softener, plus checking if acidic vent condensate is correctly flowing into household drain and eventually out to sea instead of dripping into heat exchanger causing expensive alloy to rust, crack and fail.
Other periodic maintenance includes de-liming the heat exchanger yearly, and cleaning water filter monthly.
Repair most tankless problems yourself, same day, all brands for $350.http://waterheatertimer.org/How-to-install-gas-water-heater.html