1) Rheem offers on-line chat, which you rarely see for typical tank-type water heater
2) Call local service technician, and put that person in your rolodex-wallet for future help.
Ask technician for tips and tricks for keeping unit working and needed maintenance.
They will gladly offer a yearly service.
3) Read manual several times to get accustom to yearly maintenance required for tankless water heaters. Tankless water heaters are maintenance heavy. All costs are energy.
4) Read manual several times and notice word troubleshoot appears many times.
Since tankless are complicated machines, many internal and external factors can affect operation, especially as unit ages.
5) Look at label on unit to identify exact product number. Identify same/similar product at Rheem, and become familiar with Rheem websitehttp://www.rheem.com/products/tankless_water_heaters/
6) Read other question/answers at fixya about Rheem tankless to become familiar with the tankless era
7) Install DIY tank-type gas water heater and avoid the whole headachehttp://waterheatertimer.org/How-to-install-gas-water-heater.html
8) Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.