i have a comfortmaker furnace and the unit will fire up for a few seconds and then shuts down. I don't know if there are flame sensors on the unit and one is bad or what the issue is.
i have a carrier furnace. it has exhibited this kind of behavior twice. both times it was the hot surface ignitor. this is in place of a pilot - it gets red hot and ignites the gas. they are fragile and break easily. if it breaks, gas doesn't ignite and furnace kicks off automatically. very easy to replace and they only cost about $30 or less. technician may charge over $100 to fix this very common problem. look inside your unit where the burners are. if it is anything like mine, there is a sort of "U" shaped wire thing that is close to the burners. this is the hot surface ignitor. mine has a clip-type plug on it and is mounted with a single sheet metal screw. take it out, find a replacement (just call around - the model number is usually stamped on the unit) and reinstall. just be careful not to touch the ignitor surface - oils on your fingers will cause it to break when it heats. hope this solves your problem.
forgot to mention one more thing. you can tell if this is your problem (probably) by opening up your unit and overriding the switches on the cover so you can make it run with the cover off. turn it on and let it try to ignite. if the ignitor is working, you will see it glow just before the gas begins to flow. if not, you will hear maybe a clicking sound of the gas valve, but no glowing. then the unit will turn off.
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