Furnace of thermostat issue

That number has nothing to do with the filter.

It is the Program # that shows what Program Mode ( 1 thru 4) that the t-stat is using. When it flashes, that means the preset temps for Program 3 have been overridden. By default Program 3 is the 4:00PM to

When you say the air changes from hot to cool, is the fan still running ? Normally it may run a short time as the furnace cools down, but should stop at some point. If it runs continuously, you have a different problem than the filter.

See the left side of page 6 under "Temporary Manual Override"

ut I can say it is solid. Anyway, 2 days ago I felt the air is not that hot . I put my hand and I felt it is cold or warm air not hot like before. I ch ecked the thermostat and the Filter indicator was flashing. So I changed th e filter and then went to thermostat and Press Filter for 3 second as per t he manual.

reaching the temp I set, the air turn warm to cold. I checked and I see on ly pilot flame is on, the other flames are not. I checked the thermostat an d the number below the Filter indicator is flashing 3 (by the way the therm ostat manual is here

is a number below it (in the pic the number is 2). Mine the number is flash ing 3. Could that be the problem? How can I fix that? Thank you so much.