Furnace is a Ultra V / Advantage 80 made by lennox, 9 years old.
If furnace has to bring the temp up more than 5 degrees it will run for about 15 min and then shut off with the fan still running for a few minutes.
After about 1 hour it will restart and go though the same cycle. If I turn the thermostat off and then back on the furnace will restart. I watched it and this time it gave me a code 4 on a Honeywell "smart valve ll ". The burner went off for about 5 minnuets and then came back on. Fan ran the whole time.
What do you believe is the problem?