How do you know when to replace O2 sensors?

[BBQ_HotDogs]

I have a 98 5.0L X with 70K on it. How do you know when to replace the O2 sensors without having to wait on them to go bad? I hate it when an idiot light comes on. Should I replace mine now when I did under the hood to tune it up? How many exactly are there?

Thanks in advance,
HotDogs

 

[Billy177]

there are 4. i replaced mine at 75k and noticed much better gas milage and thee engine ran better. the fronts are the only ones that effect the a/f ratio

 

[aldive]

Hook up an OBD II scanner and monitor the function of the O2 sensors; if bad or weak, replace.

 

[Glacier991]

Billy is right. The rears only monitor the job the cats do...voltage output should esentially remain flatline. Over time O2 sensors can switch more slowly, get lazy, if you will. It can be actually seen on a scanner. I suspect a lot of good O2 sensors get changed needlessly, but IF they improve fuel economy when you do it (I'd not do it BEFORE 75K unless I knew it was bad) they can pay or themelves in a short time.

 

[karl_burns]

That's weird.....

Mine went bad about 45K miles. They were replaced under warranty, but I swear they've gone bad again (78K miles) because my milage has taken a nose dive. Of course, so have the temperatures....
How much do OBD II scanners run?
Karl

 

[aldive]

Re: That's weird.....

Originally posted by karl_burns
How much do OBD II scanners run?
Karl

Take a look at these.... http://www.autoxray.com/default.htm

 

[karl_burns]

Thanks aldive

Is $120 the going rate? Or are these the pinto/cadillac of scanners?
Karl

 

[BBQ_HotDogs]

Good info here guys. Thanks. I have right now just a little over 70K on my X. I love my X and I have given her a good oil/filter change & lube every 2/3K religously. So I guess since she is now 6 years old, I should change at least the 2 around the engine. I mean I am there and will be going to give her new plugs, wires, Mac Intake, 70mm MASS, fuel filter, PCV valve, and anything else I can think of that involves a tune-up very soon. Soon as she gets out of the Stereo Shop. ;-(

HotDogs