Thermostat calls for heat, ignitor comes on, burner lights, when hot enough for the fan to come on the burner shuts off. The fan runs for awhile and the ignitor comes back on, lights the burner, runs for a short time and everything shuts off. Then the cycle starts all over again but does not reach room temperature.

Thank you, Jonathan Baus

The fan come on,, are you speaking about the smaller induction fan,which usually sits above the burners,and blows the exhaust out the vent pipes? Or the much larger blower motor that blows heat into the ducts and on into the house., to check your main blower motor,set the thermostat to fan only,this will tell you if blower is working