Quote:
Originally Posted by Shaneajanderson
O2 sensor for my odyssey is like $20. The downstream O2 sensor exists only to see if the catalytic converter is working, so actually it will throw a code if the two agree, which would indicate a catalyst failure
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The downstream does act to see if the catalyst is working, but it will also throw a code if the gasses getting past the first aren't right. It's often the case that a catalyst efficiency code is actually a failing upstream O2.