Just went over 200k (thats only 124k in miles) and the Canucklehead nation manual suggests a replace at this point.
Havent documented any changes or abnormalities in engine running or gas mileage in the years I owned/operated my 04 TL.

Should one wait for oddities to crop up first before undertaking the awkward?

The OD scanner shows no irregularities, Im not smart enuf to see the live graphed data to notice whats good or bad, and no engine warning lights since the one brief time years ago, forgot what that was about now and the code number but resolved.

 

People say a new o2 sensor may help with fuel economy as that's what is used to adjust fuel trim but i say if it ain't broke, don't fix it yet.

 

I've changed the sensors (NGK) this year and haven't noticed the change in fuel economy (160k milage)🤷
On the other hand, still have the random cell saying there is problem with one of the sensors, so it was sort of waste of money 🤑

 

Looking at the service manual found here in the forum, it looks like a couple of the swap out locations are a hassle to reach or do. Did you find it easy enough? Im trying to think what to compare it to, from doing work on the TL or almost everything u can imagine on the old Toyota Corrodda I had... (that old thing taught me SO MUCH about what to expect and prep for, what can go wrong, what looks easy in a manual that actually is a pain in the FLJDS!!HW{U*Y??)

I didnt notice any improvement when I put the top end OEM recommended NGKs in, if anything I would think that would show slight improvements on the OD or the Angrydad hack install that you can finally track/use/store that travel data via the PCMCIA slot in the trunk.
IT DID start easier and run smooth faster than before the swap mind you.
If the older O2s are working and get swapped out, and the new ones inbound are working going in, I dont imagine machine or man could tell either... cuz the sets were both working before and after.

I think will take the average suggestion of just letting them ride until things indicate.
Thanks all who answered.