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1995 Toyota 4Runner

Question: Really high CO emission following Cat/O2 sensor replacement

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grevilli, 3.0L V6, Denver, CO, August 09, 2011, 11:53
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 Rookie

I failed my emissions test with high CO and NO emissions. My mechanic checked it and replaced my Cat and O2 sensor. Check engine light came back on and mechanic tells me that the EGR sensor is the cause but to have it re-tested anyway. It failed the test again with only slightly lower CO levels (it passed all other levels).

Any ideas? Mechanic's only thought off-the-bat is the EGR, but I'm meant to understand that this does not effect CO levels. Thanks!

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  • Answer #1

    sarok August 14, 2011, 13:22
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     Enthusiast

    what were the numbers? EGR would not greatly affect CO levels. sounds like you have a malfunctioning (or unplugged) coolant temperature sensor(ECT). A faulty ECT makes the engine computer(ECM) think the engine is cold (does not warm up), this causes the ECM to run the engine in enrichment mode all the time, thus increasing CO levels (also poor fuel mileage and a dark stain on the rear bumper near the tail pipe). also, EGR operation occurs when the engine is fully warmed up. with a malfuntioning ECT, the ECM doen't see the engine warm up, thus not commanding EGR operation and increased NOx levels.

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