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Old 03-06-2022, 02:03 PM   #1
striktlysubie
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Exclamation 2015 WRX frequent CEL and fouled O2 sensors

(also posted on clubwrx.net. The more help the better)

Hi. Summary is this:
2015 Subaru WRX only had these engine mods installed up until a few month ago = Grimmspeed intake, Perrin charge pipe, and Cobb Accessport on stage1+ OTS 93 octane tune. Car ran fine. Back in December, I went to a tuner shop and had these added parts put in:
-Grimmspeed 3-port EBCS
-Process West Khanacooler top mount intercooler
-Aluminati TGV deletes
-IAG Street Series air/oil separator
-Grimmspeed equal length header
-Grimmspeed 3" catted J-pipe
-AWE Touring Edition 102mm exhaust system
+NGK spark plugs are also brand new

Car was also conservatively pro tuned. Just a few days later, I started throwing engine codes, and the car was idling poorly and bucking/misfiring. The main CEL was the "P0031" (issue with front O2 sensor). I had that pulled out, and it was full of black soot and dead. Tuner felt it was sheer coincidence due to the age of the car. There was also a minor exhaust manifold leak due to the bolts backing out, so that was tightened. So, I also bought a brand new OEM front O2 sensor, and once installed, the CEL cleared and the car ran fine. However, after that, I did not have the car for a bit due to body work being done, so have now had it back for a few weeks, and a few days ago, it began to run poorly again and smell a bit more. Then yesterday, the same CEL came on again, and removing the new O2 sensor after just 800 miles of use has that one full black again too!

Any ideas what could be wrong here, and how to diagnose it better? Parts issue? Fuse failure? Maybe a bad tune?? I can't keep blowing money on sensors.

Thanks in advance for any help provided.
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Last edited by striktlysubie; 03-06-2022 at 02:11 PM.
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Old 03-06-2022, 09:00 PM   #2
tegxsi
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P0031 is a heater circuit issue.

Check the o2 sensor connectors and wiring for issues. Maybe it was damaged during parts install
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Old 03-07-2022, 12:58 PM   #3
desertrat505
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covered in black soot is a sign of a rich condition your tuner might need to look into it.
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Old 03-08-2022, 06:03 PM   #4
striktlysubie
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tegxsi - Thanks for that info. I've also had periodic "bank 2 sensor low circuit voltage" codes in the past as well, but they usually clear quickly. Not sure if these have been related to each other. The tuner is also guessing maybe fuses are blowing or there is a bad wire/ground/connection somewhere? But we haven't checked fully yet. Electrical is beyond my area of expertise, but damage somewhere could also make sense.

desertrat505 - Also thanks. The tuner feels it's not a tune issue, but rather a mechanical issue. But, I was also suspecting a tune issue with air/fuel setup.
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Old 03-09-2022, 02:27 AM   #5
GoFrogs
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Quote:
Originally Posted by striktlysubie View Post
Also thanks. The tuner feels it's not a tune issue, but rather a mechanical issue. But, I was also suspecting a tune issue with air/fuel setup.
Ask the tuner what "mechanical" parts he thinks are creating the black soot, and why.

Valves? Lifters? Camshaft? Pistons? Injectors? Sparkplugs?

Then ask the tuner what the black soot is made of; or where does the soot come from and why does it form. The obvious answer is from the gasoline, and as mentioned above, too much gasoline.

I would be curious to see your numbers, or the numbers the tuner was getting while tuning.

I doubt an injector is stuck open. Not unheard of, but doubtful.

Were your injectors flow tested?

When the car is running (idling), what does the exhaust smell like?

Hopefully you can get it sorted.
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Old 03-09-2022, 05:28 PM   #6
tegxsi
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if the o2 heater circuit isn't working as it should that could cause false AFR readings
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Old 03-14-2022, 06:18 PM   #7
striktlysubie
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Thanks for the added replies and info guys. Still trying to sort this all out. Oddly enough, the car has run fairly fine, and has not bucked/knocked like last time, and mpg has been steady as well. However, I now have a 2nd CEL code in addition to the first one listed above: P0134 (also bad O2 sensor, or failed wiring).

GoFrogs: The car definitely smells of heavier exhaust notes, like more pungent. It could def. be the tune too. The #s I had made on the dyno after all the parts were installed were 271 HP, 278 TQ. I feel it was definitely a conservative tune, as all the parts only jumped the #s up by about 38 HP and 27 TQ, and I had expected more. But, maybe this is average??

Frustrating process so far.
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Old 03-14-2022, 07:16 PM   #8
TheClutchRider
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It's definitely the AF. Just get a new one in and have it tuned ASAP. My car works a lot better with the AF sensor unplugged than with it plugged in.

Also, make sure you are starting off with the latest stock base map for the tune. On my last legacy 2.5i i replaced 5 sensors before taking it for a reflash. After that i never had to trace another one for 2 years until i sold the car.
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Old 03-15-2022, 02:04 PM   #9
tegxsi
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Did you replace the oxygen sensor with an OEM part? Denso or NGK/NTK are ok too as long as they are not counterfeit.

Have you inspected your spark plugs? If you oxygen sensor is fouled then your spark plugs could possibly be fouled also.
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Old 03-16-2022, 12:48 PM   #10
dpetro1
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I'm not convinced that the sensors are fouled. Black soot on an o2 sensor is totally normal.

The CEL points to a wiring issue IMO especially since it came back within weeks of replacing the o2 the first time. OP mentioned not checking fuses/wiring yet. Until that is done I don't think the problem will be fixed.


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Old 03-19-2022, 07:55 AM   #11
striktlysubie
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Quote:
Originally Posted by TheClutchRider View Post
It's definitely the AF. Just get a new one in and have it tuned ASAP. My car works a lot better with the AF sensor unplugged than with it plugged in.

Also, make sure you are starting off with the latest stock base map for the tune. On my last legacy 2.5i i replaced 5 sensors before taking it for a reflash. After that i never had to trace another one for 2 years until i sold the car.
I've never heard of this recommendation before. Wouldn't changing the Cobb map back to stock still cause issues once I re-fire up the car because it was tuned with all the new aftermarket parts on it? Seems it would have more cons that pros.

tegxsi - Yes, the O2 sensor was stock from the Subaru dealership. Damn thing cost around $300, so it was legit. And I have yet to check the spark plugs, which are also brand new from NGK, and would suck if they were toast already too.

dpetro1 - You are correct. Wiring and fuses have not been fully checked yet. I tried to get back to the tuner last week for them to start doing this, but they gave me the runaround when I was there. If anyone has knowledge of common areas to check myself re: wiring and fuse locations, I'll gladly do it myself.
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Old 03-20-2022, 08:26 PM   #12
dpetro1
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Quote:
Originally Posted by striktlysubie View Post



dpetro1 - You are correct. Wiring and fuses have not been fully checked yet. I tried to get back to the tuner last week for them to start doing this, but they gave me the runaround when I was there. If anyone has knowledge of common areas to check myself re: wiring and fuse locations, I'll gladly do it myself.
Check this video for possible fuse location.

Also there is a relay for the AF sensor under the dash. Should be labeled on the cover or the owners manual

Lastly, a quick check of the o2 sensor from the manual attached.

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Old 04-02-2022, 06:54 PM   #13
striktlysubie
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^ that stuff confuses me, sorry.
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Old 12-31-2022, 03:43 PM   #14
striktlysubie
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Update: Car was worked on by a different shop, and had a full pro tune re-done. No more CELs and better power band. A lot of wiring to the ECU was checked and re-checked as well (as it seems these cars may be notorious for going haywire if the ECU is moved when work is done on spark plugs or other engine parts?). However, car is now sometimes "sputtering" when at constant light to moderate throttle, but once I blip the gas, the issue resolves for a bit. Car is not throwing any codes or CELs despite it running rough at random times. Thoughts on this now??
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Old 01-09-2023, 12:46 AM   #15
striktlysubie
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bump up
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Old 01-09-2023, 01:06 AM   #16
K3rm1tth3fr0g
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Quote:
Originally Posted by striktlysubie View Post
Update: Car was worked on by a different shop, and had a full pro tune re-done. No more CELs and better power band. A lot of wiring to the ECU was checked and re-checked as well (as it seems these cars may be notorious for going haywire if the ECU is moved when work is done on spark plugs or other engine parts?). However, car is now sometimes "sputtering" when at constant light to moderate throttle, but once I blip the gas, the issue resolves for a bit. Car is not throwing any codes or CELs despite it running rough at random times. Thoughts on this now??

Take it back to the tuner and take some logs.



Any reputable tuner will help you resolve this.
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Old 01-18-2023, 06:06 PM   #17
PollyPoger1
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Quote:
Originally Posted by striktlysubie View Post
Update: Car was worked on by a different shop, and had a full pro tune re-done. No more CELs and better power band. A lot of wiring to the ECU was checked and re-checked as well (as it seems these cars may be notorious for going haywire if the ECU is moved when work is done on spark plugs or other engine parts?). However, car is now sometimes "sputtering" when at constant light to moderate throttle, but once I blip the gas, the issue resolves for a bit. Car is not throwing any codes or CELs despite it running rough at random times. Thoughts on this now??
Maybe try replacing your ignition coils? Also have you checked the compression of your block?
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Old 02-03-2023, 04:37 PM   #18
izzi426
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I was having P0137 (bank1 sensor2) issues not too long ago, my sensor was thinking it's too lean and putting in too much fuel. How is your fuel consumption? I ran fuel additives for a tank, did a re-tune, drove car without cruise for another tank, and then checked O2 sensor. The O2 sensor looked cleaned and I noticed that the wire to O2 sensor was touching the exhaust. I remounted it and the CEL went away.
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