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o2 question


sinkrun
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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.

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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.

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