You need to, they start costing you MORE money after 100k miles or 12 years
O2 sensors generate their own voltage
0.1volt, high oxygen, "lean"
0.9volt low oxygen, "rich"
O2s use a baked on chemical to detect oxygen in the exhaust and that reaction generates the voltage, 0.1v to 0.9v
As time or exhaust flows by, the chemical gets used up
When that starts to happen voltage will, of course, start to go down
Lean and rich are not from the O2 sensors and don't mean engine is actually running lean or running rich
Computer ONLY has the upstream O2 sensors to "see" if the air/fuel mix is OK
Computer calculates OPEN TIME for each fuel injector based on engine size, RPMs, throttle position and air weight(MAF sensor)
It then looks at O2 voltage to see if that OPEN TIME has 0.45volt, good burn
If its lower voltage, "lean", then computer opens injectors a bit longer
This is where it costs you money, its a FALSE lean
As the chemical runs out the voltage goes down bit by bit, so computer adds MORE FUEL than needed so.................MPG goes down bit by bit
The computer will set a code if its calculation is off by more that 20%
Or if O2s are not changing voltage very much
But lean codes are first
And it could be something else causing lean codes but you can chase your tail around looking for a FALSE lean because O2s are old
If O2s are newer then you check other causes
Changing the older O2s will for sure get you better MPG if they are over 100k or 12 years old, so not a waste of money