I have an intermittent ignition problem with my 8 yr old Nordyne furnace. The ignition sequence proceeds as follows - The ignitor glows and the flame lights for a few seconds but does not fire the main burners. The sequence repeats up to 4 times and the furnace either fires up or locks out with an ignition failure code. It tries again an hour later or if I notice the problem, powering on and off restarts the sequence and it will usually fire up the second time around. There are no other error codes on the board so the limit circuit isn't opening and I've replaced the ignitor and flame sensor. Any ideas. It worked perfectly when an engineer looked at it, but he suggested that the burners may be corroded. Does that make sense and how can I fix it?