If it is dead, it will affect your mileage. They really aren't that hard to put in-- use an 02 sensor wrench for best results-- some stores will even loan you the tool to do it yourself-- you'll need a 1/2 drive ratchet or a breaker bar too. Alternatively you can pick up your own tool for about $10. I use Bosch sensors for 90's Fords (like the 13913 or similar). You can just cut off the plug and crimp the three wires onto the original Volvo harness. My last one was $37 and it works great.

Good luck!