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Old 03-08-2017, 08:17 PM   #1
TRebelo
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WRB

Default P0031 CEL!! Replaced But Now What!!!

2004 WRX 2.0L Turbo
Perrin Intake
Cat-Back Exhaust
Stock Everything Else

So I have the P0031 CEL so I replaced the manifold o2 (Used a Denso one) and I cleared the CEL and when I started the car it came back within seconds and light stayed on for 60+ miles, returned it and got a new one under warranty thinking I may have gotten a dud and did the same procedure and light came back within second and is still there! Now what?!? I've checked other CEL P0031 threads and everyone seems to fix there's with a new one but that's not working for me....Any suggestions or what my next step should be...
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Old 03-09-2017, 10:01 AM   #2
Charlie-III
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Silver Sleeper BK, 5MT

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Yes, common fix is replace the sensor, but there could be other issues as well, mostly wiring.

https://www.obd-codes.com/p0031

Not sure on your year/model, but some have a heater fuse and/or relay for the heater circuit related to the sensor (different from the interior heater).
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Old 03-09-2017, 10:36 AM   #3
TRebelo
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Quote:
Originally Posted by Charlie-III View Post
Yes, common fix is replace the sensor, but there could be other issues as well, mostly wiring.



https://www.obd-codes.com/p0031



Not sure on your year/model, but some have a heater fuse and/or relay for the heater circuit related to the sensor (different from the interior heater).


It's a 2004 WRX do you know what fuses or relays to check, I've read somewhere there's a brown relay into a plug behind my glove box I can check that out or even get a replacement just to eliminate it, also are there any fuses on the drivers side and under the hood that cover this? And what are the chances I have a broken wire between the o2 plug and Relay or ECU
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Old 03-09-2017, 11:06 AM   #4
TRebelo
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Anyone know the kind of relay or a part number? And when I check the voltage and resistance of the o2 plug is that with the car on or off
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Old 03-09-2017, 12:51 PM   #5
Charlie-III
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Quote:
Originally Posted by TRebelo View Post
Anyone know the kind of relay or a part number? And when I check the voltage and resistance of the o2 plug is that with the car on or off
Voltage is "key on", resistance is "key off".
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Old 03-09-2017, 02:11 PM   #6
TRebelo
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Quote:
Originally Posted by Charlie-III View Post
Voltage is "key on", resistance is "key off".


Thank you, and voltage I should read 12ish? And Resistance? Also which two do I go across to check both resistance and voltage and this is on harness plug side obviously
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Old 03-09-2017, 08:13 PM   #7
TRebelo
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So update...
I checked the front o2 plug (harness side) and I got 10.2v with the key on and for resistance I got 1.5-1.8 across them all does that sound right
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Old 03-10-2017, 03:04 AM   #8
TRebelo
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For anyone who needs it as a future reference going to trace the wiring tomorrow or just make a new harness for the front o2
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Old 03-21-2017, 12:46 PM   #9
TRebelo
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So update I checked the brown relay underneath the passenger side cleaned the connections and still nothing I tried taking apart the harness and about 6 inches down from the 4 wire o2 harness side the yellow and red wire was split into two and I stopped there don't know where there both going but I thought it would only be 4 wires all the way thru to wherever it connects
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Old 03-30-2017, 09:02 PM   #10
TRebelo
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Sooo update again I ended up replacing the main relay today, so I got a new front a/f sensor in there by Denso and replaced the main relay and check engine still comes on within 10 seconds of starting the car I checked bout 6 inches down from the o2 plug harness side and wires looked fine and then checked at ECU and wires looked fine and checked every fuse under the engine bay and every fuse under driver side and everything was 100% there I don't know what or were else to turn now
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Old 03-30-2017, 11:51 PM   #11
TRebelo
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And I don't know if this matters but when I bought the car it came with Perrin Intake, I have stock manifold stock catted uppipe, stock catted downpipe and the cat right after the downpipe where the 2nd o2 goes in has been removed with just a welded straight pipe but the o2 is installed then off to a cat-back exhaust
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Old 05-11-2017, 11:08 AM   #12
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Problem Resolved! Ended up being a bad ECU
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