sinkrun Posted January 6, 2011 Share Posted January 6, 2011 I have been told a bad o2 will cause bad mpgs so I priced them on my 99 and there around 130.00 for both. My question is if there bad would the check engine light be on or could it be off and they still be bad? Any advice would be great as I hate to waste $130.00 right now. Link to comment Share on other sites More sharing options...
Eagle Posted January 6, 2011 Share Posted January 6, 2011 Yes, a bad oxygen sensor can (and usually does) cause bad gas mileage. On a '99 (I assume you mean an XJ), the first one controls the mix to the engine ... the second one only checks to be sure the catalytic converter is working. If either one is bad, it should throw a check engine light and a code. Link to comment Share on other sites More sharing options...
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