furnace problem

My furnace, a Lennox Conservator III G16, will run and put out heat for a short time after turning it on, and then it keeps running but the heat shuts off and never comes back on. Can anyone give me some hints as to what the problem may be?
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Something is causing it to lockout. Could be a blocked exhaust, a cracked heat exchanger, insufficient air flow causing a overlimit trip. It could be any combination of the above, or something completely different. Your furnace is doing this as a safety mechanism to protect you from CO poisoning or fire depending on what the fault is. About the only thing you can check yourself is to make sure nothing is obstructing the exhaust and air intake (if equipped) on the side of your house. You can also check and make sure none of your registers or returns are blocked (make sure you don't have those ridiculous register filters, the only filter should be at the furnace).
Failing this it's time to hire a pro (and soon), a safety lockout is one situation you shouldn't ignore.
-- Steve
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