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Message: <blockquote data-quote="godflesh" data-source="post: 186556" data-attributes="member: 2276">If you have a cat converter, you should have 2, (1 pre and 1 post). The 2nd sensor monitors how well the cat cnvrtr is functioning. A failure in either 1 will illuminate check engine light etc.  If I'm not mistaken, Launch has a function where you can actually see the health of the sensors. Not just good/bad but actual fluctuations in the range of 200mv (lean) and 800mV (rich). I guess the ECU adjusts fuel/air ratio (combustion) to keep the voltage at about 400-500mV which is the optimum point (the stoichiometric air :fuel ratio). So on a healthy sensor should see a volatge in this range. I think (not too sure though) I have seen this pulse from my car's sensor on the handheld Launch tool. If the readings are out of range you could probably say that the sensor hasn't failed completely but might be heading that way. Anyone care to comment.</blockquote>