1) Before calling service technician who can test parts, open following link for Rheem resources including error codes, Rheem chat and phone number, possible manual:
If local technician cannot solve problem, Rheem can probably recommend a person.http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
2) Error codes are start-point for troubleshooting.
3) Multiple error codes can indicate PC board failure.
If your computer motherboard fails, most people feel the computer is compromised even after motherboard replacement.
4) Error code C7-76 is communication error with remote, and can include bad remote, bad wire to remote, loose wire, or bad PC board.http://waterheatertimer.org/pdf/Rheem-error-code-C7-76.pdf
5) Error C4-13 can include part failure, vent problem, oxygen depletion, and can also include PC board failure.http://waterheatertimer.org/pdf/Error-code-13.pdf
6) Tankless routine maintenance manual:http://waterheatertimer.org/pdf/Tankless-maintenance.pdf
7) Some posts about tankless concede that qualified technician was unable to repair unit.
Solve most tankless problems yourself, same day, for about $350.http://waterheatertimer.org/How-to-install-gas-water-heater.html
8) Take a moment and rate answer:
And 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.