Hello all and thanks for any help you can offer. I have a Lennox furnace that about a week ago started giving me problems. It will fire up fine, but after a few moments shuts off. It will do this several times then stop. It does a alternating slow flash between the two diag lights that indicates watchguard error. I'm at a loss of what to do. I climbed onto my roof today and looked into both of the ventilation pipes and both look clear as could be. The furnace itself seems to be igniting fine, so I don't know what else could really be the problem. My friend who lives 1 block away and has the same furnace started having a problem with his about 2 weeks ago, he paid a couple hundred dollars to a technician to come out and wipe off his sensor and now it works fine. He doesn't know where the sensor is located or what exactly the technician did aside from what he was told (he wasn't there when service was done). He told me that his wasn't even firing up though, and mine is so I don't know if that would even help me... Anyhow any advice would be appreciated... Also I checked what appeared to be the ground wire connecting to the frame and it doesn't look corroded or anything, looks fine.