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Check that you have a healthy pilot flame which is playing correctly over the thermocouple tip. If Ok check that the overheat stat is closed circuit. This is the two thick white wires that connect to the gas valve with the other end connected to OH stat on the boiler heat exchanger. For exercise...

Another common cause of your problem is the venturi in the fan system. You need to remove the collar above the fan where the air switch pipes are connected and clean out the venturi with a fine wire and an air blow. The venturi is mounted within the collar and looks like a tiny jet engine...