Rear O2 Question
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This is a discussion on Rear O2 Question within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; So, about a month ago I hit something huge in the middle of the road, ripped a hole in my ...

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    Rear O2 Question

    So, about a month ago I hit something huge in the middle of the road, ripped a hole in my downpipe, welded it back together, and everything was fine (thanks for all the advice from people on here). Since that happened, my wideband is telling me I'm running at 14.3 AFR's while cruising around town. While I realize that isn't THAT rich, it concerns me because I used to cruise at exactly 14.7/14.8.

    Since the rear O2 sensor controls the fuel input to the motor, is there a chance that maybe something broke inside of it, or it fouled somehow, or got dirty during the 3 days that my down pipe was leaking? I know 14.3 isn't going to be detrimental to the motor, but it ran perfectly for so long, now it's just ever so slightly off.

    I also thought it could just be the weather It's really cold here in Ohio..

    I've got a Cobb intake, downpipe, accessport, and a GReddy Evo 2 exhaust. Flashed to stage 2, and was running perfectly until just now. Any help? Or suggestions?

    Thanks!

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    Registered User EvoEatr's Avatar
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    That is richer.the rear o2 is simply there to compare against the front o2 as far as cat. conv. efficency and AF ratio. When its colder outside the motor will run richer, which is why you might smell a small gas odor on cold start also.

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    Thanks for the reply. I know that's richer, my question is WHY is it running richer. I'm not new to Subarus brother. I know that cars run richer in the morning as well. I appreciate the input though. And yes, the rear O2 DOES compare to the front O2, but that's how it controls the fuel input.

    It has been really really cold here. So I'm thinking the colder/denser air is making it run a tad rich. It just doesn't make much sense though. I'm wondering if I should replace the rear O2 and see if that helps any, but I'd rather not spend the money if I don't need to.

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    Moderator Donkey's Avatar
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    On the 32bit ECU's the rear O2 sensor does affect closedloop fueling.
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    Yup. So should I replace it?

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    Registered User EvoEatr's Avatar
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    The 02 sensors as you know have a pretty narrow operating range compared to an AF sensor as im sure you know, 99% of the time if the rear 02 has a problem it will throw a code for it, also the rear 02's on these cars very rarely fail, of the thousands and thousands of subarus I have worked on I think I have heard of like two having a rear 02 problem. If you are unsure as to whether to replace it or not you could plug in a decent scanner and watch the 02 sensor ...

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    That's funny, at my garage, we run into it all the time. Sometimes they're just fouled, sometimes they've been damaged, sometimes a wire got cut. I know they don't just go bad for no reason, but I've seen a pretty significant amount of damaged rear O2's.

    I'm not throwing a code, and the AFR's aren't THAT far from ideal. But, I'm stumped and so are the rest of the mechanics here. I don't just think it's the cold weather causing it to run a little richer at cruising speed. If I'm at 60mph on the highway, the AFR's will go bob around in the mid 14's, then drop down to high 13's for a second, then shoot back up to the 14's. Before, it held pretty steady in the high 14/ low 15 range, and I never had a problem.

    I'm running out of ideas. I will hook up the scanner tonight and see if I can pull any codes even though I'm not throwing a CEL. It happens sometimes, thanks EvoEatr. I appreciate the advice.

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    Registered User jd92677's Avatar
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    Quote Originally Posted by Junior2552 View Post
    That's funny, at my garage, we run into it all the time. Sometimes they're just fouled, sometimes they've been damaged, sometimes a wire got cut. I know they don't just go bad for no reason, but I've seen a pretty significant amount of damaged rear O2's.

    I'm not throwing a code, and the AFR's aren't THAT far from ideal. But, I'm stumped and so are the rest of the mechanics here. I don't just think it's the cold weather causing it to run a little richer at cruising speed. If I'm at 60mph on the highway, the AFR's will go bob around in the mid 14's, then drop down to high 13's for a second, then shoot back up to the 14's. Before, it held pretty steady in the high 14/ low 15 range, and I never had a problem.

    I'm running out of ideas. I will hook up the scanner tonight and see if I can pull any codes even though I'm not throwing a CEL. It happens sometimes, thanks EvoEatr. I appreciate the advice.
    Are you using a wideband O2? Maybe you're having a gauge issue?
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    Registered User WillieRX's Avatar
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    Quote Originally Posted by jd92677 View Post
    Are you using a wideband O2? Maybe you're having a gauge issue?
    Was gonna ask the same thing. Dont think I can trust aftermarket WB AFR gauges, ill just use my cobb for engine readings.
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    Master Baiter EJ257's Avatar
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    Quote Originally Posted by WillieRX
    Dont think I can trust aftermarket WB AFR gauges, ill just use my cobb for engine readings.
    I hope you're being facetious.
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    Quote Originally Posted by EJ257 View Post
    I hope you're being facetious.
    For that part, yes..
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    Quote Originally Posted by WillieRX View Post
    Was gonna ask the same thing. Dont think I can trust aftermarket WB AFR gauges, ill just use my cobb for engine readings.
    Haha, yeah I have an AEM UEGO. It's an incredibly accurate gauge, attached to a Bosch O2, so I'm not entirely sure why you can't trust them? Unless you were indeed being facetious haha.

    My Accessport and my gauge are in sync with one another if you read them side by side, so I highly doubt it's a gauge issue. I took a look around the front O2 (wideband of course), and it didn't look like it was loose or leaking or anything.

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    Quote Originally Posted by jd92677 View Post
    Are you using a wideband O2? Maybe you're having a gauge issue?
    The gauge seems to be fine. I'm haven't taken it out of the pod to inspect for damaged wires or anything, but everything seems to be working just the way it should

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    I guess on the other hand, it could be the cold weather. The car idles at 14.8 but as soon as I start moving is when it gets a little rich. Colder/denser air?

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    Quote Originally Posted by Junior2552 View Post
    Haha, yeah I have an AEM UEGO. It's an incredibly accurate gauge, attached to a Bosch O2, so I'm not entirely sure why you can't trust them? Unless you were indeed being facetious haha.
    It was poorly executed sarcasm on my behalf. I personally dont have a WB AFR guage, and probably wont get one because i wont be tuning the vehicle myself (if i even go beyond the OTS map). Probably the most ill get would be a boost guage and oil pressure guage. Not sure exactly why your having issues, or even if it is an issue for concern.
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