/diy/ - Do It Yourself

My Coleman Evcon Presidential furnace fan runs more than it should. The flame part of the furnace is fine, it goes on when it should and off when it should. But when the flame/heating goes off (because I turn the thermostat down or because the temperature gets warm enough), the fan continues running for many minutes. In the past the fan would stop shortly after the furnace stopped, but just as of recently, the fan runs for much longer (20 minutes? not sure) after the furnace shuts off. Why might this be and how can I fix it?
There is a manual switch for the fan so I can turn it off that way, but I want it to go back to automatically turning off after a minute instead of after 20 minutes.