Good morning,
I have a 80 plus day and night furnace.
It was working fine but when I came back from my month long trip. It wouldn't start, I went downstairs and turn the switch off and on and the engine started but there was no glowing on the ignitor and after a couple of minutes it the engine turned off again.
I did turn the switch off and on again and the same process repeated itself without any different result.

Is it the ignitor the problem?
Should I use sandpaper on it to see if the build up carbon is the reason that it doesn't start?

Thank you for your help,
Bert

You need to measure the voltage to the ignitor with a voltmeter. During the ignition process you should get 120 volts ac to the ignitor. If you do & the ignitor doesn't glow, the ignitor needs to be replaced.