Thermostat calls for heat. The little purge air blower comes on. The "glow plug" comes on. Gas does not come on. After a minute or so, the glow plug goes out. This situation may continue for some hours, with the purge air blower on and the glow plug coming on from time to time, but no heat. There is a green diagnostic light on the gas control; it is flashing bright/dim (which the manual says it does in normal operation).
I usually go through these kinds of steps to get it to respond properly: -there is an off-on switch on the gas control body; I cycle this off and on once or twice. -there is a wall switch for the whole furnace; I cycle this off and on once or twice. -I switch off and on the thermostat on the wall.
After some combination of the above steps, the furnace will properly go through its proper sequence: purge air blower, then glow plug, then the small pilot burner, then the main burners, then the main blower. It will continue to cycle properly for several cycles, perhaps days. Then it will flunk out again. Problem has occurred twice during this winter season.
I have also noticed a slight variation where the purge air blower comes on for several minutes and then stops without ignition occurring, with that sequence repeating several times (maybe 4-5 times) after which the gas ignites.
Salient facts: natural gas furnace; 80,000 btu/hr Honeywell thermostat and gas control Houston, TX about 7 years old; as far as I know, no maintenance has ever been done furnace is in a closet in my one story home there is no smell of gas during the problem A label on the unit "installer's instruction manual" shows: Consolidated Industries Lafayette, IN Mod # MBA 080 NH4R Serial # 990917039