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Check engine light

Tiny
RONALD FRANKLIN
  • MEMBER
  • 1999 OLDSMOBILE 88
  • 3.8L
  • V8
  • 2WD
  • AUTOMATIC
  • 11,300 MILES
My 88 Anniversary Edition has the check engine light on po420, catalyst system efficiency below threshold bank 1. I replaced both up and down stream 02 sensors also ran seafoam through the brake booster vacuum. Reset computer but check engine light came on after a few miles. My sticker expires this month. Any help would be greatly appreciated.
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Wednesday, February 14th, 2018 AT 3:31 PM

1 Reply

Tiny
CARADIODOC
  • MECHANIC
  • 33,910 POSTS
That fault code is detected by analyzing the readings from properly-working oxygen sensors. Replacing the sensors does not correct the problem they are detecting. This is set when the catalytic converter is no longer cleaning up the exhaust gas. Your mechanic will likely want to perform a few tests to verify what the engine computer found, but the repair will involve replacing the catalytic converter.
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Wednesday, February 14th, 2018 AT 3:55 PM

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