It sounds like the limit switch is tripping due to an overheating condition.

I assume the filter is clean and not too restrictive. However even the best filter will restrict some air flow. Try running the furnace temporarily as a test with no filter and see if the problem still occurs or the frequency of faults changes. If it does then you don't have sufficient return air flow.

Mike,

Thanks for the post. I had a new 1" Flanders MERV 8 pleated filter installed when I first noticed the problem. I replaced it with one of the cheap 1" Flanders fiberglass flat filters as a test. Of course, using the 1" fiberglass ("rock catcher") filters is almost like using no filter!

All of the registers in this four-level split-level (with finished basement) are open, and clear, and we have an excellent return air duct system design - all are open and clear.

The problem still remains.

The thought of the limit switch tripping did cross the tech's mind (as I mentioned in my original post). It's time for a call to the Lennox dealer for a call back.

Here in s.calif we can call sdg&e and they will come out free of charge and check our heaters,the stove,hot water heater,and also smoke.and carbon monxide detectors.We get ours checked every year,as i trust them to tell me the truth more so than the appliance guys.If it's something small like a thermacouple and they have one on their truck,they will fix it,no charge.If it's something else that we have to call a repairman for,they will tell us exactly what we need.
Kathi

On the G20 series (mine is the Lennox G20Q3E-75-1), and using the Robertshaw "ignition controller" (called "electronic controller" in the original Lennox manual), if the pilot light goes out while the main valve (MV) and the blower are running, then the unit will attempt to relight the pilot light. This will also happen if the there is a flame sensor fault (because the unit will read that the pilot light is off). This process ensures that the main valve is not pumping gas through unlit burners (when answering the call for heat). Running the manufacturer's System Checkout Procedures (laid out in Robertshaw 1-963 Rev D) can help to identify the issue.

Given the problems laid out above, I would want to test to see if the flame sensor is faulty (e.g., are sufficient microamps being sent from the electrode (while under an active flame) to the flame sensor connector ("Sense") on the ignitor controller?). The documentation mention above has the troubleshooting process laid out. Best of luck to the remaining owners of these units.