Hi, I have an Armstrong furnace (not sure of the model, I can probably get it later though) with a Rodgers digital thermostat, and I've been having trouble with it lately. It seems to happen mostly in the late evening, but basically the thermostat shows that the house is colder than what it's set to and the furnace does not come on. If I flip the switch off and on I hear the whoosh of the flames but it's off and on quickly and the blower does not come on. If I mess with it, flipping it off and on, I'll eventually get it to work. I'm thinking that the thermostat is bad but I've also come across similar questions that seemed to point more towards a relay of some sort. I can't say if it affects the AC or not since it's cold here and it's only been happening recently. I've made sure the thermostat is clean and changed the batteries multiple times. Any help would be appreciated, thanks in advance.

Follow-up: The model is Ultra V Tech 80, #G1N80AU075012A-1, made by Armstrong Air. The serial number is 1602E10533 and it's less than 10 years old. When I open the top panel there is a blue box above the gas line with a green LED light and an on/off switch, I'd imagine this is the gas valve. The light is blinking, 6 flashes a pause then 2 more flashes. According to the manual it's a "Soft Lockout Type 2 (flame sense lost during run)." This seems to indicate trouble with the flame sensor. I managed to find them online and they are pretty inexpensive, however, it was noted that they rarely good bad and it was likely just dirty. I removed it and cleaned it with some super fine sand paper. I popped it back in and have not had trouble since. It's only been half a day, but it looks promising. I wanted to post here just in case someone else has similar issues.