Why Do Furnaces Break On the Coldest Holiday Night of the Year?

[Note the cross post. Follow-ups set to alt.home.repair]
Ah, what luck. My furnace quit working last night. This is a long rant and a "poor me" post, not a request for help. So if you're not to inclined to point and laugh at the clueless, you might not want to read on.
I thought the trouble was the thermostat. The Lux 500 had given me trouble many times before; it showed the low battery warning even with fresh batteries literally if I touched it wrong when adjusting the temperature.
A few days ago I noticed that it sometimes the Lux thermostat let the temperature fall five degrees below the set point before the furnace (Ruud Silhouette II) ran. I'd have to turn it to off and then back on, then it would run ok for a few days. I was concerned about the Lux crapping out entirely, so I picked up a Honeywell RTH110B at Lowes. I didn't get around to installing it.
Last night (New Years Day) at around 10 PM the temperature was in the upper teens, the coldest night we'd seen this year in Louisville. My furnace stopped running. I assumed it was the thermostat.
The symptoms were: if I turned the thermostat to "off" then back to "heat" the furnace would begin to start. I'd hear the whine of the power vent. The the igniter (it's a pilot-less furnace) would glow, and then the natural gas would start. The gas ignited fine, all burners looked perfect, but it ran for about five seconds then chopped off abruptly. This cycle repeated two or three times then nothing until I turned the thermostat off and then on again.
It was getting cold in the house, so I decided that now was a great time to replace the thermostat. :-) I turned off the breaker to the furnace, and swapped the Honeywell for the Lux.
It was the first time I swapped a thermostat. I was disappointed at the flimsiness of the connections of the Honeywell. The screws were too damn tiny, and the plastic back flexed so bad I feared it would break as I tightened the terminal screws. But nothing broke, and I completed the swap with apparent success.
Hoping I hadn't bought a dud thermostat, I turned the power back on and then turned the Honeywell to heat. The power vent started and I figured I had a winner. Good thing, as it took an hour to swap the thermostat and the house was even colder.
Guess what, the furnace did the same thing it did with the old Lux. Start, light three times, each time quitting after five or ten seconds.
At this point I was really frustrated not to mention worried. I had no means of alternate heat. No kerosene heater, no electric radiator. Stupid of me, but I'd been in the house since May and it was just one more thing I had meant to do. With visions of frozen pipes in my head, I wondered if the 24 hour Wal-Mart was open at 11:30 PM on New Year's night.
And I had to be at work on Wednesday, so I was looking at missing work to let in a repairman. And would I be lucky enough to get a repairman before the pipes burst?
Before I left to find out if the Wal-Mart was open, I decided to try my luck with a Google search of the furnace symptoms. Fifteen minutes later, I knew that it probably the flame sensor. I even found photos of what it looked like. Plus some model specific information that my furnace has eyes, that like the flame sensor, need periodic cleaning.
Another hour and a half later, the sensor was lightly sanded with fine paper, and the eyes were gently wiped with a paper towel. I also wiped the sandpaper grit off the flame sensor. It is now three hours since the furnace failed. My house is now in the mid 50's and if it weren't for the stress and the exercise I'd be cold.
Power back on, thermostat on. The furnace starts, lights ... and stays lit and produces most welcome warmth.
The Internet. Secret lover. Fixer of furnaces at all hours and on holidays too. Is there anything it cannot do?
Tony Sivori

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