I am a recent convert to this way of thinking. I acquired my current vehicle after it had already spent the first 100K miles of its life as a city police squad car, and while there were no O2-related DTCs or faults in the system, I decided to replace the upstream sensors with new ones anyway. I started a few threads on various different automotive forums declaring my intent and inviting discussion on whether or not I was being prudent, foolish, wasteful, etc.
Not surprisingly, the feedback I received from every thread was mostly the same regurgitated, out-dated opinions, most of which stated the sensors should be left in until failure. Even less surprising, I was provided with NO evidence, other than anecdotal, that could convince me the O2 sensors were somehow immune to wearing out gradually like other sensors on the car. So I went ahead and did the unthinkable - I purchased non-OE parts from a local parts house** and replaced the upstream sensors despite their seemingly robust health.
Well, let me tell you, just because the O2 sensors seem to be behaving fine, and there are no active (or inactive) O2 sensor faults stored in the PCM, that does NOT mean they don't need to be replaced. The tips of the sensors I pulled out of mine were covered in cement-looking deposits that I couldn't remove without a stiff wire brush or file. I don't know what caused them or how much they were affecting the sensors' operation, but there is no chance they weren't affecting the sensors' operation to some extent. Maybe not enough to cause it to throw a fault, not yet anyway, but those senors were not doing their job like they did when new, that's all there is to it.
By the time I finally got around to doing the job, the engine had about 130K miles it, and I had estimated the sensors should fall into the 100K mile maintenance service category, so while I may have been correct to think they needed it at 100K miles, it is likely they were losing some functionality well before that. YMMV obviously.
I would love to have some Before and After datalogs showing real proof that replacing my sensors before they failed was the right thing to do, but I don't. I tried to record some datalogs for that very purpose, but I could not get the PIDs necessary to log data showing the O2 sensors' output in an understandable way, so I gave up.
I can say my average MPGs for city/hiway mixed driving went from 18 MPG at time of replacement to 20 MPG now. That wasn't an immediate jump, but more of a gradual one that took several weeks to come on. No other performance mods have been performed in the interim, heck, the air filter is even still the same one. The average ambient air temp has increased significantly since I changed them out, so I would have expected a decrease by a few MPGs actually by now. But that is not the case - it is burning the fuel at a slightly more efficient rate than with the old sensors, and the sensors are the only variable that has changed (besides weather).
Go ahead and do it, it certainly cannot hurt to have new sensors in there - that's my position on this subject.
Nuke
** -I will note that I decided not to buy the sensors from the dealership parts house mostly due to their price point. The Mopar branded O2 sensors were over $80 each from them, so I would have had to spend close to $200 for parts alone to go with OE sensors. Instead of doing that, I went with the NTK branded equivalents purchased at a local automotive shop (O'Reilly Auto), and spent almost $40 each for those. So basically half the price of the Mopar branded ones. I could have ordered them off Amazon and saved even more money if I truly wanted to.
I did have the benefit of knowing beforehand that the Mopar branded sensors are made by NGK and re-branded as Mopar, or that's the conclusion I came to after doing some research on the matter. I also uncovered, during that research, that the aftermarket brand NTK is NGK-made-and-rebranded as well. So basically I believed (and still do) that the NTK O2 sensors were the same as the Mopar ones, and that was what pushed me over the edge to go with them.
