Error code 76 is communication problem between remote control and tankless unit.
http://waterheatertimer.org/pdf/Rheem-error-code-C7-76.pdf
Multiple error codes could indicate that main PC board is bad.
Before calling service technician, open following link for troubleshooting resources including link to Rheem chat, manuals, error codes, etc.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
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