My furnace won't complete the ignition cycle.

York Furnace
Model: P3UCD12N07601A
Updraft

Here are the steps when the furnace attempts to ignite:
1. Exhaust fan and igniter turn on.
2. A little while later the gas comes on and ignites.
3. After about 2 seconds, the igniter goes off (starts to look less red).
4. About 1 second later, the gas goes off and the igniter comes back on. The igniter is only off for about a second so it never completely cools.
5. Repeat steps 2-4 several times.
6. Igniter goes off and exhaust fan continues to run. Eventually, I notice the house is cooling off, go to investigate, and find the furnace in this state.

I first noticed this a couple of weeks ago. If I turned the furnace off, waited 15 seconds or so, and turn it back on, it would ignite normally. Now it is happening more and more frequently. At the moment, it is happening 100% of the time.

What should I be looking at to identify the problem?

Try cleaning the flame sensor with steel wool or sand paper. They got a coating on the steel and that causes this exact type of problem.

Cleaned the flame sensor and so far so good. There is lots of info on the web about this. I just didn't know what key words to search for. Thanks!

Hope it keeps running.