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2004 Mitsubishi Endeavor P0421

My check engine light has been coming off and on for 2 years. Code P0421. Had the 02 sensors changed last week, light went off. Drove for 4 days, light came back on (stayed on) for the past 2 days. Light went off this morning, chanced DEQ because tags expire in a week. IT PASSED! What gives? Hiccup in my cars system? Miracle?

Should have the catylitic converter checked. Could be starting to malfunction.

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