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.
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.
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