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Check Engine Code P0421 on 2003 Hyundai Santa Fe

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What does code P0421 indicate that I should have repaired?
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(1) Comments
A code will never point to one item as needing replacement, it takes a diagnosis of the related systems to determine the fault. Here are some things to check for that code:
A code P0421 may mean that one or more of the following has happened:
* The catalytic converter is no longer functioning properly
* An oxygen sensor is not reading (functioning) properly
* A spark plug is fouled up