My 1998 F150 4.6 has almost 220,000 miles on it. I have replaced lots of parts over the years as trouble codes popped up or as needed. But I have never replaced the factory oxygen sensors. I bought my truck with 50,000 miles so I don't think the previous owner replaced them.

I have never had a O2 trouble code pop up and fuel mileage has always been a consistent 15-16 MPG. So my question is this: is it necessary to replace them even when everything seems okay, or should I wait till fuel economy declines, performance suffers, or an obvious O2 code flashes?

 

I'd replace them.. I'm replacing mine in August and I'll have about 93k or so by then. I'm **** though haha

 

With that many miles, you better at least crack the rears(downstreams) loose while she's running. DON'T force those too much, - very easy to trash the bungs on the way out. Add more heat to just the bungs at the same time,- if you have to. Once the bungs are screwed, it's a different ballgame that can cost you more than double.

The upstreams are always easier to loosen. Passenger side can be a PITA to manipulate, unless you have the correct Ford O2 sensor socket, but will loosen with less torque effort than the rears once in position.