2000 Grand Mercury Marquis
Yes all four of the sensors are needed by the computer. If you mean that you are having a cat back system put on then there will be no issue since their mounting bungs will still be intact.
well........yeah- get headers with those O2 portals on 'em-
The O2 sensors make sure your engine is not running to rich or too lean. Your gas mileage will suffer without them.
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