Gas furnace won't stay on! Help!

I have a Lennox gas furnace and when the thermostat calls for heat, the blower starts, and the furnace ignites then burns for 10 seconds and the burners turn off....then after 20 seconds, the burners turn on again...and burn for 10 seconds and click off again....never burning long enough to create hot air to be cycled through the ducts. Sometimes it will try to get going 5 times before it gives up all together. Sometimes it will take 3 or 4 attempts to keep the burners on and they will keep burning and create hot air blowing through the ducts and vents. What is up with all these false starts?
Freezing sometimes!
thanks in advance!
Nate

 

Mine did this, but eventually would get hot enough to kick on the blower. I had to clean the flame sensor.POOF! no more of THAT problem.

 

Thanks...I will try to do that.
CAn you offer some direction on how to clean the flame sensor?
How did you do that?
thanks so much.

 

Sounds like the same problem the girlfriends furnace had... You don't specify the model but the gf's is a g24m.
Userguide here:
http://www.lennox.com/pdfs/manuals/L...M%20Manual.pdf

Service manual here:
http://www.ultimatehvac.com/LinkedDocs/Lennox/9723b.pdf

I'm suspecting that you're having something similar to the watchguard diagnostic 6.2 on page 34 of the following service manual...

http://www.ultimatehvac.com/LinkedDocs/Lennox/9723b.pdf

 

The patch to view the service manual helped...thanks...
I watched the LED lights and they suggested that it was the Watchguard Diagnostic.
Next step is to figure out what do do about that.
Any thoughts?
IF cleaning the flame sensor is a key...how do I do that?
thanks again if more advice rolls in.
It is very appreciated.