I have a 15 year old 90% efficient ICP natural gas furnace (NUG9100FHB1) which intermittantly doesn't ignite. Sometimes it works fine. When it doesn't, I observe exhaust blower starting. Then the ignitor lights, a yellowish flame is visible, then it goes out. (This takes about 4-5 seconds.) This repeats in 10 second intervals until the main burner ignites (big blue flame) and the blower turns on. Sometimes, the main burner ignites, and then goes out before the blower turns on. I also believe that the main burner has gone out while the blower has been on.

The exhaust blower went out about two years ago and was replaced. The ignitor was replaced last spring. I also purchased a backup ignitor and put that in, which didn't change the above problem.

Try cleaning the flame senser

The flame sensor is integrated with the ignitor on this furnace (1009524). I put my spare ignitor in, which is new, so it should be clean.

I've uploaded the wiring diagram, if that is helpful to anyone.

Okay, so your flame is actually sensed through the spark wire, not all that uncommon on older spark furnaces. If replacing the spark assembly didn't fix it, most times it is actually the ignition control that is faulty. However, keep in mind that this is only an educated guess. Can't say for sure without putting hands and testing equiptment to furnace. Good luck, and please do let us know the outcome.