Upon arrival tankless unit was very loudly operating when fan would activate, and was having issues with hot water consistency. Performed diagnostic of heater including removal and cleaning of fan. A good amount of debris was retrieved from fan. Fan returned to normal function after cleaning. Checked the condition of flow and temperature sensors as well as the burner chamber and wiring harnesses. They looked to be in decent condition. While cleaning the intake screen hard water build was noticed. Hard water build up also broke loose and clogged the kitchen faucet upon operation of isolation valves. Discussed the flushing of the unit as possible solution. Though the outcome could not be determined. Unit will be replaced.