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Q: CODE: PO 421 on 2000 Mitsubishi Eclipse

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I got a code #421 warm up catalyst below threshold
(BANK 1) CAN SOMEONE TELL ME WHAT THAT IS
(1) Answer
It means it is likely that you have a catalytic converter that has stopped being effective and needs replaced.

But before you do that -- verify the operation by testing it. The efficiency of a catalytic converter is measured by sensors before and after it. A problem could exist there.
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