Heater Shuts Off- filter new

[NoDummy]

I have a five yr old Lennox Gas Heater. It has been shutting itself off intermittently. The burner will ignite and heat up. Then when the blower starts, it will run approx 30 seconds or so, and then stop. Some time later everything will work properly - the heater will kick on and run fine.

The filter is brand new and changed often ( monthly ). I am wondering what I should check. Is it some sort of safety switch or what?

[NoDummy]

I see I have already commited Hari-kari, please forgive me. The model number is:

Lennox # G24M375

[kdog]

what is shutting off exactly- the therm calls for heat,the flue fan starts,the burner ignites,and once the plenum is heated,the blower starts- then what?  does the blower shut off,does the burner go out, does the flue fan stop.

[NoDummy]

The blower shuts off and I assumed the the burner and everything else did too. I will try to examine it further. Since it is intermittant, I haven't hung out by it and given it a visual. I will do that and report back.

Thanks.