Navien npe 249a2 installed in January 2023 and working fine until three weeks ago. Upon request for hot water, flow rate only reads 0.5 gpm even though actual flow is more like 3-4. Hot temp out is only a temp rise of about 5 degrees (75deg f). No error codes. Sometimes upon hot water request it just keeps cycling between startup and post purge. Sometimes it reads 3-4 gpm and actually makes hot water, but this is becoming very rare. Again no error codes ever. I’m doubtful it is the flow sensor (don’t worry, plumber is replacing it when the part comes) because it is a strange failure to read 0.5 gpm. I know the impeller is turning, there is no gunk or scale in the screens or flow sensor. Could it be anything else?