Heat performance tuneup times one system. Inspect blower, evaporator and ductwork. Tightened all wire connections. In second stage the heater was emitting 2 to 3 ppm of carbon monoxide. I advise clients that it is the beginning of the end for their furnace and possibly their system. It is 17 years old with original motor and circuitboard. Check thermostat for proper operations. Recommendation today is to change out the furnace. When systems are above 15 years old they are very unpredictable. One minute they will be running fine in the next minute they will not be running.