Hello,

My furnace recently started this. It goes thru the normal cycle but
intermittently will continue burning after the blower shuts down. It
will burn for maybe 30-45 seconds or so after shut down... then burners
off. Then after about 30 seconds the blower will come back on for about
30 seconds.

It just had its bi annual checkup with the local HVAC company about 30
days ago and the only notation they made was possible low batteries in
the thermostat. I replaced those shortly after the service because I
noticed this problem on the first use of the furnace, and unfortunately
have procrastinated finding the root cause since.