I was called out to look at a Pentair MasterTemp 400 pool heater in Newport Beach today. After running for about 15 minutes, the display showed E05. Diagnostics registered stack flu sensor error. The stack flue sensor shuts the heater off if exhaust gasses exceed 500 F. There can be several causes for this error including bad wiring connections and water flow issues.
In this case, the thermal regulator had already been changed out, so unless the heat exchanger was plugged up (possible, but not likely) or the pump/filter had an issue- which I saw no sign of- I wouldn’t suspect a water flow problem. What I did notice as soon as I walked in the yard, was the tiny gas meter (pictured). This meter fed not only the 1.5″ line to the 400k btu Mastertemp heater, but a 1.5″ line into the newly remodeled house.
I recommended calling the gas company to change out the meter, which they do for FREE. Just for fun, I checked the resistance on the stack flue sensor, but it was within tolerance. There may be sooting or other combustion issues, but they probably arise from inadequate gas supply volume or pressure. Best to get that fixed first.