Rheem RTG 53 PVN with fault codes C4 or C7

Question

My tankless water heater has been increasingly noisy over the past year (its almost 2 years old). Suddenly, it won't make hot water for very long. The control flashes fault codes of C4 then 76 (alternating) until I unplugged it, waited and then reconnected it and tried to use it again. Now it flashes C7 76 and won't produce any more hot water. What is the problem?

Thanks

Rich W