Do any other digits appear when you look at the LCD panel? Most Rheem Tankless water heaters will have a two-digit number in addition to the C2 code. Some of the error codes may be in your water heater manual.
http://www.tanklesshotwaterguide.ca/about/rheem-error-codes-tankless-systems/ has some of these listed.
For example: C2 11 indicates a failure to light the gas burner (for gas/propane) units. This could be do to a shut-off valve that wasn't completely open. There could also be air in the gas line or insufficient pressure. There might also be a problem with the ignition/wiring.
A C2 00 code indicates that the burner was on for more than 60 minutes. For this code, turn off all open hot water faucets, including a dishwasher or clothes washer. Turn off the water heater. Turn off or remove recirculation pumps. Wait an hour for the unit to cool and reset the error code. Try the unit again.
Please add a comment with the full error code and the model number of the water heater for additional information.
I hope this helps.
Cindy Wells
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