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?

The message is a 'warning' that your CAT might be bad. Usually it is because of failing O2 sensors, which fail because the system is always rich due to several vacuum leaks.

In other words, work backwards. Find and fix any vacuum leaks, then if the O2 sensors still have problems, replace the front sensor.

Your CAT is probably still good.

Please use only 89 octane or higher gasoline.


Klaus
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Volvos should be saved from the junk yards, not donated.

Thank you for this very detailed and cogent way to proceed to analyze this far from straight forward problem. The shop that scanned my car suggested replacing the CAT for $1500. Given the absence of any symptoms, it didn't make sense to me. I am so grateful to have a viable plan. I love my Volvo and want to maintain it properly, but on the other hand, do not want to pay for unnecessary repairs.

Thanks again. You have been very helpful.

The next time the check engine light comes on, stop at most any autoparts store, they will scan all of the codes for free. Write them down so you don't forget, in case there are more than one, and let us know.

Your car is now old enough to start having vacuum line leaks.

Klaus
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Volvos should be saved from the junk yards, not donated.