Pentair MasterTemp 400 Heater Won't Fire

[Kit]

I noticed that the heater would not fire when the water temperature fell below the set temperature in either the spa mode or the general filtration mode. In the general filtration mode, the heater blower did not even try to run & there was no error code on the heater display panel. When I select the spa mode, the heater blower tried to run, but just cycled on & off every second. (Obviously the heater itself would not fire until until the blower was running). I looked at the back of the heater display panel & noticed that both the AFS & the LED11 lights flickered on & off in the spa mode. Since I had a similar problem about a year & a half ago, I removed the thermal regulator. The "stem" was completely rotted. So I ordered a new thermal regulator.

However, given the above symptoms, are there other components that might be contributing to the heater blower not running & the heater itself not firing? Thank you so much for any help you can offer. Truly appreciated.

 

[ajw22]

Read about AFS errors in Pentair MasterTemp Heaters - Further Reading

 

[Kit]

Thank you, Allen. I am hoping the problem is the obviously-bad thermal regulator. However, I will check to be sure that the heater blower switch is not contributing to the problem.

 

[ajw22]

Bad thermal regulator does not keep the air blower from running at startup.

 

[Kit]

Makes sense. Update: I removed & checked the 2 plastic tubes connected to the AFS. All was fine. I removed & cleaned the 2 electrical wire terminals on the AFS. They looked fine, but I cleaned them anyway. I removed the white plastic air input tube & inspected for any air flow obstructions. Nothing. Then just on a hunch, I navigated to Settings > Heat > Pool & noticed that Heat had been set to OFF. I noticed the same thing for the Spa. Subsequently I reset Heat to HEATER. Now the blower runs & the heater seems to work fine. So maybe the problem was simply that the heat setting was set to OFF. Fingers crossed.