The Google machine to the rescue.

[fatboyz]

After reading those recent threads about furnaces quitting through the holidays and the cost!, mine quit through the night. It would try to light then shut down. Did a Google search and found that it was most likely the flame Sensor. Even had a picture where it was. Took it out and ran down to the local plumbing shop and for nine bucks we're back in business again!

[Kim473]

Probably could have just cleaned it and it would work. Flame sensors are like that. You got lucky that it didn't happen xmas eve or day.