O2 sensor

My Trooper's mileage seems to be getting worse. It has 120K miles on the clock, and AFAIK oxygen sensors have never been replaced (been driving it since 98K). I'm thinking about replacing O2 sensors - how long exactly are
those supposed to last? Also, Alldata shows sensor's pigtail being kinda round, but all sensors I've been able to find online have rectangular plug type. Does that mean I'll have to splice the wires and re-use existing plug?
Advice appreciated! Peter

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.