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Does the heat function still work just the blower wont shut off?
The Carrier line had a board that a relay stuck on..but one of the safety features makes the same symptom. So does the unit heat?
Ok, so it is on safety...was/is the filter plugged?
The 2 main things are fusible link bad or furnace on limit. I know you said the furnace is new. But was it when this problem started.
There are 2 different directions to take and try not to waste your time. Was the filter new when the problem started?
The furnace is not new.It is 22 years old.
The filter was not brand new,about 2 weeks old,but clean.
see page 11 here -
The fusible link is a soft solder fuse in the wire that is above the burner. If you get flame rollout from a bad exchanger it melts the inside and cuts power. That is one choice....the other is an open limit, but if filter was kept clean it is unlikely. The limit can be tested by removing the 2 wires from the flat rectangle plate behind the gas valve and touching the wires together thus bypassing the switch.
The final choice if the previous 2 items are ok is a bad circuit board.