I have a 2001 V70 XC with 122,000 miles. About two years ago, the check engine light came on and the Volvo service advisor said that I might need a new catalytic converter. However, he suggested that they first install a new sensor, and then see what happens. Since then, in the past two years, the check engine light has intermittently come on, has stayed on for a month or two at a time, and then goes off - again for a month or two at a time. Volvo service again says that I need a new catalytic converter. However, if the converter is bad, then why does the check engine light periodically stay off for an extended period? Any thoughts?

Thank you.