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I second tobster. The mostly blocked cat will allow the pipe to fill up, then backpressure will dirty up the O2 and finally get back to the head. Putting your hand over the tip and smelling it might indicate at lot.
Another wild guess for the vacuum leak theory is the brake booster has an internal tear in it, or the EGR is sticking open. Try taking off and blocking all the unessesary hoses.
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Meanwhile other things are still happening.
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