That displays when the ignition control fails to sense adequate flame current at one or more of the burners.
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Off the top of my head, here are some possibilities:
Faulty flame sensing circuit.
Faulty ignition control.
Faulty burner/blower assembly.
Failed wire connection.
Gas turned off.
Maybe something else I can’t think of right now.
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If it’s due to the steam generator burner not sensing, then I’ve also encountered water inundating the burner heat exchanger due to a crack or corrosion of the exchanger.
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To isolate and troubleshoot your problem, you’ll need to enter technician level programming to function test the burners and components individually.