What's the right rear O2 sensor?

[Garrison]

I'm trying to find the right rear O2 sensor for my 98 2.5L but it seems like there is a LOT of them out there.

A little nudge in the right direction prease? Thanks

http://www.partsgeek.com/catalog/1998/s ... ensor.html (http://www.partsgeek.com/catalog/1998/subaru/legacy/fuel_injection/oxygen_sensor.html)

[mike-tracy]

The downstream would be the rear one, since there's only 2 on 2nd gens. Is that what you're asking? Both Denso and Bosch make good sensors.

[Garrison]

Mike,

Yeah that's what I'm asking. There seem to be a few different kinds, 3-wire; 4-wire, different makes and models for the same year range, etc.

[mlitz]

The NTK# is 24520

[Garrison]

So I do want the 4 wire? Thanks!

Is there one brand that's better than the others &/or one I should avoid?

[Huffer]

Denso or Bosch are fine - spend the extra coin and get one that has the plug already wired in - get under your car to make sure you have the right plug/wire combo before you order.
Take that opportunity to soak down the O2 sensor with PB blaster or similar to make removal a little easier.

[anothernord]

Yep, the rear is a 4-wire plug, for some reason. The plug n' play ones are nice for sure, but they're more expensive. If you're cheap like me, the Universal Bosch sensors work fine, just don't use their weird plug thing.

[Garrison]

Yep, the rear is a 4-wire plug, for some reason. The plug n' play ones are nice for sure, but they're more expensive. If you're cheap like me, the Universal Bosch sensors work fine, just don't use their weird plug thing.

Jared,

What's your take on all this? You think this could be causing my stumble?

I'm going to be replacing a couple of injectors as well to help with this nonsense.

[mlitz]

the rear o2 sensor really does not do much other then monitor cat function so it shouldn't have any thing to do with a stumble.

[Huffer]

The rear O2 measures the level of different exhaust gasses and temp and correlates with with readings from the front O2. If either sensor isn't working, it will cause the computer to dump more fuel into the engine to prevent running lean. This is only a temporary fix as too rich of a mixture will actually cause the catalytic converter to breakdown quicker. Each of the O2 sensors has a heater in it to bring it up to temp quicker, but I suspect the rear O2 has a 4th wire to allow for cross correlation of measurements.

To say that it doesn't do anything is fairly dangerous.

[mlitz]

I never said it didn't do anything, I said all it does is monitors the catalytic converter function. So if the cat is not doing its job that would mean a: the cat(s) is bad or b: the front O2 sensor is bad and dumping more fuel then needed in to the exahust and the cat(s) cant keep up.

http://repairpal.com/rear-oxygen-sensor-replacement


The purpose of the rear oxygen sensor is to monitor the oxygen content of the exhaust gases leaving the catalytic converter. This gives the emission system a way to monitor the operation and efficiency of the catalytic converters. The rear oxygen sensor checks several things in the post–catalytic converter gases. It measures the amount of oxygen, or peak threshold, in the exhaust gases and follows how fast the amount of oxygen changes. On some vehicles, the rear oxygen sensor is mounted midway in the catalytic converter to measure how efficiently the reduction bed convertes NOx by checking the amount of oxygen that is released back into the exhaust flow.

The rear O2 sensor has nothing to do with adjusting AFRs all it does is monitor the catalytic converter function.

[Huffer]

^^ that's the point - without the rear O2 the front one can't do it's job. So it DOES play a role in the overall performance of the car. Think about it.

[Garrison]

I just crawled under the car to triple check before I order and my rear sensor has only 3 wires. Now I'm confused...

[02_Legacy]

Ok break down of o2 sensors:
1 wire = no heating element grounds through exhaust
2 wire = no heating element signal and ground wire
3 wire = heating wire, signal wire and shared ground wire
4 wire = signal wire and signal ground, heating wire and heating ground

@Huffer the post cat 02 sensor can be totally disconnected, not function at all, ect and it will have NO effect what so ever on A/F ratios, it only will set a code for either rear 02 or cat efficiency. The rear 02 cannot determine A/F ratios becasue a properly working cat should store Oxygen to be released as H20 and CO2 as HC's pass through the cat. Even when properly working the rear 02 WILL NOT affect A/F ratios, its ONLY job is to monitor how well the cat is working.

[Garrison]

So then I'm not looking to replace my rear O2 sensor, I'm looking to replace the front O2 sensor?

[Huffer]

Well, I got a code for the front O2 sensor, but I'm replacing both of them with Denso direct plug replacements. Funnily enough, the rear sensor has a 4 pin plug, but only 3 wires.

[Garrison]

Yeah, same here. I thought that was weird.

But going back to something you said in another thread; something about it not receiving the right signal. I figure its either the front O2 sensor or the injectors. Can't imagine what else could cause it.

[02_Legacy]

If you are still chasing the misfire problem that you posted in a different thread then yes, your up-stream/front 02 sensor is the one you will want to replace.
opposedforces has 3 part numbers for the front 02 depending on mfg date.
Part number 22690AA220 for '95, December — '97, July or '96, July — '98, May
Part number 22690AA290 for '95, December — '97, July or '96, July — '98, May or '97, October — '99, May
Part number 22690AA43A for '97, October — '99, May

I guess this still would not help you too much becasue there are 2 options for every mfg date. It may narrow it down a little though.

[Garrison]

If you are still chasing the misfire problem that you posted in a different thread then yes, your up-stream/front 02 sensor is the one you will want to replace.
opposedforces has 3 part numbers for the front 02 depending on mfg date.
Part number 22690AA220 for '95, December — '97, July or '96, July — '98, May
Part number 22690AA290 for '95, December — '97, July or '96, July — '98, May or '97, October — '99, May
Part number 22690AA43A for '97, October — '99, May

I guess this still would not help you too much becasue there are 2 options for every mfg date. It may narrow it down a little though.

Holy shit that's a confusing list.
Surely they can't be that different among the same year? Mine was manufactured in 10/97 - which looks like all three choices apply. :-(

[Garrison]

Twatever the consequences, I went with an O2 sensor that said it was going to fit my MY.
We shall see how it goes! Will report back with results.

[02_Legacy]

Front or rear?
I did some part number cross referencing on napa, here are the results, 22690AA290 is a 3 prong plug so that is not the correct one. 22690AA43A is a 4-wire sensor. Which leaves 22690AA220. This number cross-references with:
BOSH 13445
NGK 24520
DENSO 2343088

Hope this helps and I hope you got the right one.

Edit: note that these numbers reference front 02 sensors.

[Garrison]

Yessir, I got a front O2 sensor. We'll see if she fits

[Garrison]

Update:

I ended up with an OBX front O2 sensor, with 3-wires and 3-plugs. It was a direct plug and play (always nice); and went in without much fuss.
As far as the effects go, I undid the "-" battery terminal and soaked the TB with about 1/4 can of TB-cleaner spray and let it sit for about 10 minutes. When I finally got it restarted, I gave it a little while to idle, and then took it for a drive. When I got back, I gave it another few minutes of idle to allow it to settle whilst I listened to the exhaust tone (which is how I normally tell if it misses).

Guess what? No misses and a nice smooth idle :grin:

Now; I'm not taking this as gospel (if you will), but I have a road trip to Johnson City soon, to meet with my AF recruiter. We shall see if the traditional CEL pops up while cruising. Usually it will do so with about 30 minutes of driving at a constant speed; so an hour or so should serve as a nice follow up test.

[jjay03]

The rear 02 is just for monitoring it does not affect the front o2 or the fuel in any way under normal conditions. I like to use the direct fit Denso o2 sensors and I get them off amazon. In fact my last car I removed the rear 02's and tapped up the plugs and then turned the DTC's off with the tuning program I had since I didnt have cats. I then used one of the rear o2 bungs for my wide band sensor.