Access your product manual.
I cannot find a product manual.
Some major company, like Rheem, makes this computer that heats water.

Call service technician to repair the unit.
Copy following link for overview of type of problems found with tankless computers.
For example tankless computers require continual maintenance on yearly and monthly basis to prolong expected life of product.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

If tankless computer is older than 5 years, then assume it is like your 5-year old laptop computer.

Save problems, and repair the unit yourself, same day, by installing tank-type water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Gene
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If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7