Scott Corey-reid posted an answer for this Pentair Master Temp 400K BTU Natural Gas Heater Low Nox ASME 460775:
"This is not an error code. 128 r13 is the normal programming or software version for your..."
Scott Corey-reid posted an answer for this Rheem M206A 206K BTU Pool and Spa Propane Gas Heater:
"thermal regulator or internal bypass needs to be replaced"