Hi Bob, sorry for the problem.
Error codes are a starting point for diagnosing problems, and the same error on one machine can have different cause and solution than another machine.
Open following link for Rheem troubleshooting resources that will be handy to keep on file.http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
Generally C7 is remote cannot communicate with tankless unit.
Your remote is connected by wire back to the heater.
You can turn power off, remove remote wires from connection terminals on tankless, turn power on, check if tankless works correctly.
If tankless works, then problem is the remote control, or wiring, or parts inside tankless.
Chances are good, the wires are not the problem.
New remote costs $99, but that may not be the problem, so instead of guessing, you should call a service technician.
Steps above could restore functionality and water heater will produce hot water at default temperature.
Also read manual for advisories, troubleshooting error codes, and required maintenance.
For example water filter should be cleaned monthly and heat exchanger de-limed yearly.
Tankless are maintenance heavy, and parts and service combined with maintenance makes these units a risky investment compared with expected $1-8 monthly savings from owning gas tankless.
Reading the manual can save some expense, and the link above is a good resource for people wanting to hold down costs.
Repair most tankless problems yourself, same day, any brand for about $330.http://waterheatertimer.org/How-to-install-gas-water-heater.html
Upvote the help.
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