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Q: catalytic convertor failure on 2003 Mitsubishi Montero Sport

1st converter replaced 2 years ago R/S.
R/S replaced again 1 year ago. Check engine light back on again in no time.
Time for emission testing. Light on. codes P0421, P0431. Exhaust shop explains my need to replace both converters.
Would a converter fail this quickly?
What else would cause failure code to be present
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The only thing I can think of is either too much oil burning or coolant getting into the exhaust through the combustion chambers. I think you need to go to a Mitsu specialist, he may have some insight on seeing the car. Normally catalytic converters last for many years, but oil and coolant will damage them.
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