During our visit, the customers Rheem tankless water heater received a full maintenance. All aerators and filers were cleaned orf debris. The unit was flushed with cooking grade, white distilled vinegar for 1 hour. Previously Rheem tech support recommended to flush the unit to resolve the P1 code. After fully servicing the unit, the code was still present. Considering the lack of maintenance over the 9 years the unit has been in use, I recommend that unit gets replaced.