probably a plugged secondary, I replace lots of these. Carrier covers these under warranty right now, but many companies won't do the work because the $ isn't that good, and it appears that carrier is catching on that the problem is also with the lack of proper start-up and/or maintenance. Everyone that I do a secondary replacement on has either been over-fired or not maintained properly. You're also in the gray area as to whether your primaries are gone too. I find that around 5 years, only the secondary is gone, at at 10 years both primary and secondary are gone.