Lennox / Evcon Furnace Shuts Off Frequently

[conflitm]

I have a Lennox C23-46-1 that runs fine for an hour or so and then shuts off before it reaches the requested temperature and the circuit board LED flashes 4 times indicated "Limit Circuit Open." If I cycle power to it then it runs again for an hour or so and shuts off. It will also turn itself back on if left alone for about an hour. The circuit board says it is an Evcon model #BGU12520A. I already tried cleaning the flame sensor. Any help is greatly appreciated! Thank you.

[conflitm]

I also replaced the air filter and it will turn back on also by backing down the thermostat below the current temperature (so you hear it click) and then turning it back up past the current temperature (you hear it click again).

Thanks

[conflitm]

It's been in for 16 years. There are no duct restrictions. I'm not a mechanic/electrician by any means so I'm not sure what is meant by ohming out the switches, checking the capacitor, etc. I did open up the exhaust duct and there is no buildup.

[conflitm]

I'm pretty sure it is turning off because it is getting too hot inside the furnace. It seems to run slightly longer if the door is open on the back of the unit. Can you please tell me what to look for in this case? Is there 2 separate fans (1 to blow the hot air into the house and 1 to clear out the chamber)? Perhaps the one to clear out the chamber is malfunctioning? Thanks!

[conflitm]

There are 4 limit switches in series and I think they are working as they should. Could it be because I have all the basement heat vents blocked off? The rest of the house (2 floors) are all open. I don't have a need to heat the basement. Also, how hot should the exhaust duct get? Mine gets too hot to touch which makes me think there could be some obstruction in it on the roof (birds nest maybe). Thanks!