My gas furnace is cycling on and the main "fire" is lit for 2 seconds and it quickly goes back to pilot light. I'm thinking this is most likely a furnace problem even though I had it inspected in January. The thermostat is from the 90s (Honeywell Chronotherm III) but I doubt that's the problem, even though they don't last forever. I wouldn't want to pay for a service call when all I needed to do was replace the thermostat. Changing batteries in the thermostat did not fix this.