Check engine light

1999 OLDSMOBILE 88
11,300 MILES • 3.8L • V8 • 2WD • AUTOMATIC
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RONALD FRANKLIN
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  • 14 POSTS
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.
Feb 14, 2018 at 3:31 PM
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CARADIODOC
  • ✓ASE Certified Mechanic
  • 34,321 POSTS
  • ASE Certified Mechanic
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.
Feb 14, 2018 at 3:55 PM
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