I have a 1980 carrier gas furnace - I changed the filter and immediately smelled something burning - looked inside and found it was the transformer- at this point the fan stayed on with no heat- replaced the circuit board and transformer - now I turn thermostat up and the pilot lights, heating element lights, fan turns on and within a minute or so no pilot or heating element. It will continue to cycle like this. When I disconnect the lower fan limit(not sure of name) that plugs into plenum the fan turns on
Please help - I'm at my wits end
Thanks

EXACT model number of unit.