Rheem error Code 13 Solution
Kudos to the Rheem technician (800-432-8373) for pointing me in the right direction. After 1 year of flawless operation we began experiencing sporadic "13" error codes causing us to reset 1-2 times per day. Once I got past the original tech suggesting a multitude of lame reasons, I was transferred to a real tech who suggested examining the FLAME RODS (thermocouples) for deposits and use either steel wool or fine grit sandpaper to remove them if present.
Sure enough, one of them showed deposits and now we're back to operating reliably. These tankless units appear to be over-engineered and prone to sensor issues after prolonged use which leaves consumers with a bad feeling about their unreliability. I greatly suspect examining and cleaning the FLAME RODS would solve the majority of code "13" issues.
I've had to clean them with my furnace every few years and found them more accessible and a simple fix. Accessing them inside of the tankless unit is not as simple, but proved just as effective. :)