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I have a 1998 Chevy P/U, 1500, 5.7. My scan tool reads PO420 - which is catalyst system efficiency below threshold (bank 1) Can you explain what that means and how to repair it?
Thank You - Robert Norris
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The pcm compares the reading of the pre cat senssor and the post cat sensor and determine if the cat is working as designed, to truly compare these functions a scope or graphing scanner should be used, there are other reasons for this code other than just a bad cat. For a thorough explanation I would google the code, lots of in depth info out there.

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