Master temp 400


In The Industry
Sep 28, 2019
Turn the heater on fan turns on then immediately goes to service mode. Water flow is good, gas pressure good, changed board, changed air flow switch still goes to service light immediately. Any ideas.


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Check for dirt or insects at the air metering orifice.

From Pentair MasterTemp Heaters - Further Reading

The air flow switch (AFS) senses the pressure drop across the air metering orifice. As soon as there is sufficient air flow,the AFS closes, closing the circuit to the hot surface igniter (HSI), which ignites the fuel mixture. On a call for heat, the blower and HSI are energized. In about 20 seconds, the gas valve opens and ignition occurs. The HSI then switches to asensing mode and monitors the flame.


TFP Expert
Mar 2, 2011
You might have a leak in the heat exchanger.

You can remove one of the bottom bolts that go to the manifold to see if water comes out.