There are multiple causes for a furnace not to ignite, due to built-in safety factors. This is the first line of defense. In other words, if something is wrong you just don't go into operational mode and possible cause a fire of explosion.
My furnace was not a Frigidaire, but when one of the safety thermostats became non-operatinal, the furnace would not ignite. Low gas pressure, low air flow due to a clogged filter are just some of the things that can trigger your furnace to stay in shut-down mode, as well as a fault in the sensors that check for theses condition.. A faulty component in the smart board, almost anything. I believe some of the newer furnaces even have a CO sensor in the airstream.
Your problem of non-ignition, is probably the symptom that has the highest number of possible causes.
Alwayas start with the most simple of causes (or the easiest to check) and work you way up. Your manufacturer's literature may have a fault tree.
Be very careful. It's not like working on a heat pump, where there is much less possiblity of personal danger.
If your furnace is not wanting to ignite, there is a very good reason. If you inadvertently override the safety systems and get it to ignite, when the initial problem has not been solved, there is a very real danger of death or severe injury.
