I have a Lennox Merrit Series model #G33-30B-F which is 4 years old. The burners will ignite and run for approx. 2-5 seconds and then shut off. The furnace will try to light 3 times and then kicks off into the the "Burners Failed to Ignite" mode. If I shut the furnace off at teh main switch and then turn it back on again it will run fine until the next cycle needs to start. Any ideas on what is causing this and how to fix it? The filter was just changed and the dust and dirt has been blown out of the furnaces. Thanks.