Lennox Furnace Run-on

I have a Lennox natural gas furnace, model G24M3/4-100A-2 with electronic ignition. The furnace was installed new in 1994, and I see from a service log that the burners and inducer were serviced in 2002, and the inducer was replaced one month later, after the servicing. We live in Canada, where this furnace is heating for 5 months of the year. I don't know if I have a problem or not,but the blower motor seems to run on longer than the burn period. So, you end up with unwanted cool air blowing out of the heat registers, and a blower motor that is running an electric bill up, unnecessarily. Here is a run-down of a complete cycle: 1. thermostat sends signal to furnace to heat home 2. flue fan starts, and runs for 45 seconds 3. burners ignite, and burn continuously 4. blower motor starts 45 seconds after burners ignite 5. thermostat sends signal to furnace that ambient temperature is 1 deg warmer than set desired temperature 6. flue fan stops and burners shut off simultaneously 7. blower motor continues to run for 3 minutes 8. blower motor shuts off I was told by a furnace repairman, over the phone, that the problem is most likely the pressure-switch, and it should be replaced. This pressure-switch is an easy DIY replacement, and would cost me $40, but before I go ahead and get a non-returnable switch, and find out that I still have the blower running longer than the burn cycle, I want to know if this is the normal operation for this furnace. And, is there anything else I should be looking for, before I have to get a repairman to come have a look for $60/hr.
Does anyone have any experience with these furnaces, and what would you suggest?

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