My furnace just blows cold air. I checked on thermostat. There is
nothing wrong. I took off the panel on furnace and pushed white button.
The two green Led Indication Lights are on with(DS1-- Slow flash, a
flash/per second, DS2--on). According to the label on the panel it
means "primary/secondary limit switch open". I have no clue what it
means and what the problem is. I notice there is no gas coming out and
the electronic ignition does not turn red. But It blows cold air.

My furnace model is Lennox Merit Series G40UH-69D-155-01. It was
installed Apr. 2001.

Here are my questions.
1) What is the possible problem?
2) What does "primary/secondary limit switch open" mean?
3) Is it still under warranty? The Lennox website says "5-Year Limited
Warranty on covered components."

Thanks in Advance.
Shiling