My recent experience may help: I could hear the ignition "sizzle", but furnace would not start. Resetting occasionally worked. First service call was replacing gas flapper and overall check. Failed 2 days later, same symptoms. Second service call replaced ignition module. Failed 2 days later. Third service call was a long one, pulled lots of stuff apart. Determined original ignition module OK and put it back. FINALLY, thought to check the spark plug. That was it all along, 20 minutes and $20 would have done it the first time. It is beyond me that, when something witha spark plug fails to ignite that it isn't one of the first things checked!