Quote:
Originally Posted by Ecky
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.
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That makes sense. So if the upstream is running things rich, it won't all get burned up in the cat and this will trip the downstream? I've never heard of that before, but most of the stuff I've worked on was pre-OBD2, and/or hasn't had cat problems.