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Discussion Starter · #1 ·
Hello, this is literally my first post and I hope I'm doing it in the right place. I bought a used 2011 base model WRX a few months ago and I've been loving it. Last week the CEL lit up immediately after a fill up so I got my scanner to scan it. The code was P0137 which indicates an issue with the rear O2 sensor. I ordered a replacement sensor and planned to replace it. Since then the CEL went off so I procrastinated the repair. The light came on again (same code), again after a fill up. Since then the turned off again.

My question is this. If the O2 sensor is bad wouldn't the CEL stay on? or at least every time I started the engine? Also, why is there such a strong correlation between fill ups and the light coming on? I don't want to replace the sensor if that's not the issue. Has anyone had a similar experience, or know what the issue may be.

By the way I did replace the gas cap already thinking that may have been the issue. I have also inspected the wiring harness to the best of my knowledge, which is VERY limited.

Thanks guys and gals.
 

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There are separate codes for the O2 sensors and the fuel evap things like the gas cap. It is an easy swap since you have the part - grab a loaner sensor removal tool at autozone and hit it with some PB blaster. You can also try cleaning the contacts. Intermittent stuff is a bitch, but if you don't want it to keep popping off I'd start with the thing the code directly referred to.

Also welcome to the club. I'm not sure why your post got flagged by the spam filter - please PM me or other mods if you need a post approved again.
 

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Discussion Starter · #3 ·
Thanks for the reply mycologist. Do you think the sensor could have anything to do with fueling? Or was it just a coincidence the CEL came on right after fueling twice?

Thanks for the kind welcome. I have many plans for my WRX so I'll be around here frequently.
 

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Mine was wonky for a while and intermittent then eventually came on full time. I would replace the sensor and see where that goes.

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No I don't think it does. It is the sensor or the wiring to it.
 

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Discussion Starter · #7 ·
That is a different code than mine rgjoe but the symptoms sound almost identical. I just unplugged the harness and sensor and cleaned their contacts. I would have just replaced the sensor but I just found out my jack is broken, because of course it is, FML. Unfortunately (and fortunately) the CEL was off when I did this anyway. I figure I'll just drive like normal, watch out for the CEL to come back and pick up a new jack. If it comes back I'll swap the sensor and write up my findings here.
 

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The rear O2 sensor handles fueling you need to fix it. It isn't optional.

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The ECU may make some minor corrections from the rear O2 sensor to protect cats or something, and I believe a few tables were added to AP maps to prevent that. The fueling is set by the front O2 primarily though.

I don't think you need that rear sensor at all with a stage II tune.
 

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The ECU may make some minor corrections from the rear O2 sensor to protect cats or something, and I believe a few tables were added to AP maps to prevent that. The fueling is set by the front O2 primarily though.

I don't think you need that rear sensor at all with a stage II tune.
Per Cobb and my experience it made a difference when I replaced it. I had a knock on cylinder 4 and when I replaced the rear O2 it went away even after having the codes blocked. Maybe someone who knows the 32bit ECU can help me understand better, but that is the experience I had. As far as I'm aware the only thing the ots tune disabled is only the catalyst code, but it's function of fuel trim adjustment was maintained.

I don't want to guide op wrong, but I fought that O2 for ages and it sucks.

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Per Cobb and my experience it made a difference when I replaced it. I had a knock on cylinder 4 and when I replaced the rear O2 it went away even after having the codes blocked. Maybe someone who knows the 32bit ECU can help me understand better, but that is the experience I had. As far as I'm aware the only thing the ots tune disabled is only the catalyst code, but it's function of fuel trim adjustment was maintained.

I don't want to guide op wrong, but I fought that O2 for ages and it sucks.

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Interesting (and sorry to hear that) - I had seen the last post here and thought it was resolved but I haven't had to mess with mine:
Cobb Accesstuner Do NOT remove your rear O2 sensor - Page 12 - Subaru Impreza WRX STI Forums: IWSTI.com

Edit - I see now Cobb were talking about having added the tables not having made the necessary corrections to them.
 

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Yeah, I can't imagine what it adds to the game that the front o2 can't handle but it does in fact do something. It's irritating though because I found the cheap replacements don't work well, and your basically stuck with either 250 for the factory part or 120 for the denso part.

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Discussion Starter · #13 · (Edited)
The light came back on today. Bought a new jack and Ill be replacing it when I get some free time. I bought the Bosch sensor from amazon for $50. Hopefully it'll last.

My understanding of the rear O2 sensor is it's for mixture control. If O2 levels are too high it makes the mixture more rich and vice versa. A lean mixture is more dangerous and damaging to the engine so it stays rich while the O2 sensor is malfunctioning. The ECU needs this data for driving at different elevations and air densities. This sensor being bad basically means significantly less MPG for the driver and a slightly "flooded" ignition. Otherwise car runs fine. I found out this CEL code could also be due to a crack in the manifold or exhaust. A crack could cause an excess of oxygen at the sensor above it's threshold causing it to throw the code/CEL.

Other question for you guys. What do you guys use as a front/center jack point? Or should I just be less lazy and jack up one side at a time?
 

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Discussion Starter · #16 ·
Yea, looking at the sensor half of it was caked with carbon. I probably could have fixed the issue by cleaning it. I'm keeping the old one in case I have any issues with the new one. Unfortunately, being 6'4" and 300 lbs makes squirming around under the car very difficult with the 14" jack I have. If you can get enough room for your body under the car the repair is super easy.
 

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Discussion Starter · #17 ·
Unfortunately our CEL friend reared it's ugly head again today, P0137 again. At this point it could be the wiring harness, a faulty replacement sensor or a cracked manifold. I think I'm going to hand this one over to the professionals to get it fixed. I just hope my warranty covers whatever is wrong.
 

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Check the exhaust gaskets.

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Discussion Starter · #20 ·
Update! I ended up going to the shop. By the time I got there the code had cleared again. The tech checked the O2 sensor, torqued it in tighter, and checked continuity for the sensor wires and connections. He then told me absolutely nothing is wrong with the car and I wouldn't get any codes again. I dove about a three hundred miles without issue and all the sudden I got a new code P2096 .

P2096 Is listed as Post Catalyst Fuel Trim System Too Lean. Any ideas? I think I'll take it back to the shop. From what I've read this may a condition that I should not drive at all during. If my engine is running lean I don't want to risk it. Hopefully they give me a discount for not finding anything in the first place.
 
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