do it yourself and you can do it for about250 - 280 you'll need to know if u have to buy 1 or both oxygen sensors(always a good idea preferrably to replace both at same time.) http://www.autozone.com for DIY
A quick thought -- I have had terrible luck installing aftermarket oxygen sensors on a Jaguar. Worth the extra bucks to get the original equipment from the dealer, in my opinion.
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I agree on a Jag or real high end vehicles especially. But its a cheaper way to go if moneys tight and you have the tools.