I need to replace the upstream (front) O2 sensor on my 99 OBS. It looks pretty stinkin tight under there. Do I have to drop the exhaust to get a wrench around it or remove the heatshield or anything, or can it be done without removing/dropping anything else on the car? I wanna know before getting underneath it and all dirty.

 

 

 

Also, I hate to buy a new O2 sensor and find out that isn't what is throwing the code. I have a 99 OBW parts car that has the same 3-wire plug on its sensor. The car had not thrown any codes, so I thought I would swap on that O2 sensor and see if it fixed the issue temporarily (until it went bad as well, being that it is a higher mileage engine). Are there any issues with swapping in a used O2 sensor? (other than it being used and of unkown life left)

Has the CEL come on in your 99 OBS? Did you have the codes read? If not, I suggest you do that before replacing any parts.

 

If you want to confirm that it is the O2 sensor, you can try swapping O2 sensors like you mentioned, and/or resetting the ECU and seeing if the CEL comes right back.

Yes, it threw a code. I don't recall the code offhand (written down at my house) but it was the one commonly thrown for O2. My car has 125,000 miles on it. I cleared the ECU and drove a week before it came back on again.

 

I couldn't see how to get the thing out without dropping the exhaust, so that's what I did. The O2 sensor off my parts '00 OBS is totally different, so that's not an option. My other parts car is in a field and I don't really feel like crawling under it and trying to get the exhaust dropped, and it has over 150,000 so that's a lot of time for one that might go bad itself in the next 10,000. I'm going to try my luck with a universal sensor and splice it. Local dealership can't have one in stock until Monday.

 

The one I pulled out of my car looked a LOT more "fouled" than the one I already had out of the 2000, even though it had more miles than mine.

 

 

EDIT: I haven't noticed much difference in gas mileage, not that this thing ever did very well anyway. Throttle response seemed somewhat inconsistent though, but not really anything someone other than me would probably notice. I average 23-24 mpg in this car. '99 2.2 AT.

Edited July 21, 201015 yr by JT95

Don't know what your time schedule is, JT95, but last time I was looking for one Amazon had a great price (plus free shipping) on Bosch universal O2 sensors. Comes with a splice kit that worked quite well.

 

Good luck.

Don't know what your time schedule is, JT95, but last time I was looking for one Amazon had a great price (plus free shipping) on Bosch universal O2 sensors. Comes with a splice kit that worked quite well.

 

Good luck.

 

Thanks.

 

I just got the Bosch universal kit from Advance for $60. Had I planned ahead a little better, I would have ordered online.

just a quick note on the "splice in" O2 sensors - if there is a problem with the wiring end of the sensor, chances are the splice in unit will not fix the problem.

 

I went and got the Bosch direct fit replacement for my car - it was a few bucks more than the splice in unit, but I dont have to worry about the wiring itself...(and the wires on my old sensor were looking a little rough...)

 

just a thought...

Well, I swapped in the Bosch O2 sensor as well as a replacement clockspring. It was very nice to drive my car again with no lights on the instrument cluster. Drove the car all day, and then in the evening, about 250 miles after the swap, the Check Engine light came back on. Grrrrrrrrr. So, apparently, the O2 sensor didn't fix anything.

 

I suppose I will try to make sure there isn't a small exhaust leak anywhere. I hate playing detective work with these codes.

 

 

No one ever told me--is there a way to replace these front O2 sensors without having to drop the exhaust? I couldn't see any other way to reach it. Has anyone used an offset O2 socket?

dont know how/where it is placed on the 99's, but on my 90 all i needed was a 22mm offset boxend wrench - was pretty easy actually. Looked like it could be a pain, but it really wasnt. mine sits right in the crotch of the "Y" going into the cat.

 

go get that code read again - it may, or may not be the O2 sensor again - best to make sure.

 

Seems I have read where the newer models (with 2 sensors) have issues being too strict on the tolerances between front & back and an anti-fouler can usually take care of the problem - perhaps do a search on that to get more info...

I too would recommend a directly replacement O2 sensor vs. the splice ones.

 

On my 97 Impreza I can replace the O2 sensor with a little maneuvering from under the car. It's not easy, but can be done.

Thanks for the feedback. I'll go ahead and replace the rear O2 sensor, as I don't think they are very expensive. Those are recommended to be replaced at certain tune-up points in the car's life anyway, aren't they? I've hit 125k. If that doesn't do the trick, I will do the anti-fouler trick and look into cat replacement w/ a low mileage used unit down the road.

 

Are the factory replacement O2 sensors made by Bosch?

 

 

My previous Sube was a 95 Legacy. I put 235,000 on the odometer before selling it. Had one check engine light the whole time I drove it. (Knock sensor.) It had factory O2 and cat still too.

honestly, i have no idea if the newer ones are made by bosch or not - the old one on my 90 was a bosch (only have one & it was 20 yrs old...) - had more vanes in it than the aftermarket bosch replacement, but it seems to be perfectly happy with the new one - even increased my fuel mileage by a couple miles per gallon.

 

seems to me it is recommended to replace them every 100K or so...but i usually go with the "if it aint broke, dont fix it" theory on that kind of stuff.

oem o2 sensors are denso and bosch. Use direct fit only. the splices mess with the voltage...extra impedance.

2001 forester.

 

On this car, I reached the front O2 sensor by leaning over the passenger side on top of car.

I am willing to bet the store that the code you are getting is for catalytic inefficiency. You will NEVER fix that without replacing the cats with subaru cats or using the sntifouler work around. The cat is not bad just not working perfectly. I had the same situation and used an aftermarket cat. The coed went away for 3 months and then returned. What a waste of money. I even replaced both O2 sensors and the coed still came back. A master mechanic (subie) told me that it's a very common problem with all the crappy gas out there. He said the subie cats are much more robust but even he wouldn't replace them for what subaru wants. He told me to just deal with the CEL and check the codes every so often to see if a new one popped up.

 

Good Luck