O2 Sensor Question

[Wharfrat]

Hey all. New to the site and my '98 318ti; only had it about 2 months; 96k on it. Used to have a 5 series many years ago but I love the 318ti better as it's unique & a lot more responsive. I'm 6' 4", 280 lbs. & fit in that little rascal just fine.

CEL came on about 3 weeks ago with some rattling underneath the car. Was hoping for something minor but you know how that goes. Catalytic converter bit the dust. :frown: Had it replaced as well as adding a Flowmaster exhaust. CEL went out plus performance is a little bit better on the low end and it sounds real nice now. :biggrin:

Was jumping on it a little bit :smile: in 3rd gear & CEL light came back on. Took it back to the shop & had it scanned. O2 sensor needs to be replaced which is no big surprise since it's around 100k miles.

Read the great write up on how O2's work; there's one in front & back of the Cat, etc. Learned a lot. The big question is: Which O2 sensor needs to be replaced; both of them, one of them, get it scanned to id which one, or what?

I'm not a gear head but I like to know how to tell what's what. I do a little of my own wrenching on a very small scale but that's it. Sounds like this fix could be within the boundaries of my limited mechanical skills. Basically, I just fly them, I don't build them.

Any advice, info, or plain old opinions would be most appreciated.

Thanks

[Kraln]

Get it scanned to tell which one. The one that adjusts the AF ratio is the one in front of the cat, the one that tells the OBC if your cat is bad is the one behind it.

You could theorhetically pull them out, hook them up to a multimeter, and blowtorch them, but it's probably just easier to take it to a shop.

[aceyx]

Kraln is right; the code will tell you which sensor failed. If you're at 100k, you may just want to replace both.

Replacing the sensors is an easy job. Use a bit of penetrating spray first, then the special socket (to clear the wire) to pull it. Most places rent this tool for free. You can use a crescent wrench, but then you can't torque the new one down correctly (37 ft-lbs / 50NM). I'd say you can just crank on it, but that varies too much among people.

[Wharfrat]

Thanks all for the assistance. It's pretty much what I expected to hear. Doesn't sound like it's a bad idea to possibly go ahead & replace both sensors since going to the trouble to do it in the first place. That way it's over & done with & can hopefully move on to other maint/upgrade tasks.

Thoughts?