When the EPA started to crack down on wood stove emissions, the early technology utilized catalytic converters to help clean up the emissions from the stove. Over time these converters can become brittle. Sometimes it gets as bad as when the chimney sweep pulls it out of its housing, the converter simply crumbles and falls apart. These can be replaced (obviously.)
I wanted to post this so that you would not be taken by surprise if this happens. I have cleaned wood stoves that had worn converters, and no matter how gentle I was in removing the converter, it just fell apart.