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Thread: PCM Code P0420

  1. #1
    Join Date
    Feb 2008

    Question PCM Code P0420

    PCM Code P0420 states that the probable cause is "Catalyst system efficiency below threshold". Nowhere in the manual for a Honda, EX, 1998 auto does it make a reference to this code. What are the possible defects relative to this code? Thanks for your help. (Please tell me it's not the converter.)

  2. #2
    Join Date
    Jan 2007


    mvp275 - Most likely it is the converter. The downstream O2 sensor(s) read the exhaust oxygen content coming out of the cat(s) and if it is not right the cat isn't doing its job properly and the code will be set.

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