I have an intermittent problem with my check engine light. 3 years duration. It comes on in the winter but goes off occaissionally. In spring and summer it is mostly off. When scanned, the diagnosis is : catalytic converter, low threshold. At this point, I don't remember the code #. It passed emissions in September of 2011 when the light was off. No symptoms of a bad converter. No sulphur smell. No rattling of fractured ceramic. Can anyone shed light on this perplexing problem?