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Discussion in 'Member Introductions' started by Yukon Ron, Jan 18, 2014.

  1. Yukon Ron

    Yukon Ron New Member

    Hi, I was searching the web for some info on a 0420 code and saw this site. I have a 2002 GMC Yukon since new, with 180,000 miles on it. Runs great but the mileage has been dropping the last couple of years. It used to get 17 mpg and now it is only 13 mpg. I am considering changing the oxygen sensors. I know the code is only for the drivers side but with the mileage I was thinking of just doing them all at once. Any thoughts or opinions would be great. Thanks
  2. MrShorty

    MrShorty Epic Member Staff Member 5+ Years 1000 Posts

    P0420 points to a fault in the post-cat O2 sensor for the driver's bank. It is possible that this code points to a failing O2 sensor (and, by extension, perhaps the other three O2 sensors are also getting slow). It could also point to a failing cat. Back when I got a similar code (P0430), I started by switching the two post-cat sensors. If the fault is the one O2 sensor, then the code should follow the O2 sensor. If you continue to get a P0420, that would lean more towards a bad cat.
  3. Yukon Ron

    Yukon Ron New Member

    Thanks I'll give that a try. Doesn't a failing cat drop performance to a point I would notice or does it happen so slowly I might not?
  4. MrShorty

    MrShorty Epic Member Staff Member 5+ Years 1000 Posts

    When I got this code on mine, I asked a mechanic friend about this, especially since at the time my Explorer (six years older) still had the original cats. He suggested that the OBD-2 post-cat sensors can be so sensitive to a failing cat that the OBD-2 system catches a failing cat long before actual driveability symptoms are noticeable.
  5. Yukon Ron

    Yukon Ron New Member

    Thanks for the info.

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