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Discussion Starter #1
I decided to finally address this CEL this afternoon and am disappointed to find that what I thought was the problem wasn't the problem :(

The 0031 error refers to "Front 02 heater circuit" which I have heard from many sources that it refers to the front 02 sensor. So i did the popular resistor mod and bypassed the sensor with a 2200 resistor. no luck.

Searching on Nasioc, I found someone who said that it actually has to do with the car's heater or the electronics associated with it. He said it could be the heater element in the sensor (?), a damaged sensor wiring harness, or a bad ECU....

My heater seems to work fine. The only thing is, when I come to a stop after the engine has been warm for a while, it smells like something is burning inside my engine bay.

Anyone here have the same problem or any ideas as to how this can be fixed?

Thanks!
 

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The 0031 error refers to "Front 02 heater circuit" which I have heard from many sources that it refers to the front 02 sensor.

My heater seems to work fine.

Thanks!
:rotfl:

Oxygen sensors have a heater built in that warms them up to optimal operating temperature. The code refers to a problem with that circuit, not your cars heater. You can check resistance of the heater circuit, unplugged and at room temp of course, to start your diagnosis. you'll need a diagram and a DVOM.
 

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Discussion Starter #3
Oh.... hehe, ok, what he was talking about now makes sense. Thanks for the clarification.

Do you know where I can go to get the wiring diagrams?
 

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Not really, you might be able to find something on nabisco. I use Alldata at work, but that doesnt help you right now.
 

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Discussion Starter #5
I'm on the hunt...

Can the heating element be replaced independently of the sensor?

... I also just realized the "resistor mod" I spoke of was for the temperature sensor, not the 02 sensor. It's a completely different all together.
 

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No, the heater is built in to the O2 sensor.

The resistor mod was to simulate the rear O2 sensor signal eliminating the subsequent code from removing cats. I wouldnt recommend a resistor to replace a temp sensor.

Happy hunting.
 

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Discussion Starter #7
Temp sensor = egt sensor, the one that you remove when you put a catless uppipe in.

Ok, so I found some wiring diagrams. I'm a little confused by the fact that they only list one o2 sensor for the WRX. Is the rear o2 sensor something else? I know the front o2 measures A/F as well, so I was looking for something along those lines in the document I have and there isn't anything.

The o2 sensor i'm looking at has a white wire that goes to pin 65 on the ECU. is that the correct o2 sensor?
 

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I really dont know specifics about that system. Typically, the sensor is at fault with these types of codes. You can look quickly for any rubbed wires, loose or poor connections and so on. The sensors are not that much, or that difficult to replace. Testing it is the right way, but replacing with a "known good" part is faster, if you have the means.
 

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Discussion Starter #9
So I'm not sure, but I think the doc I was looking at was for an older impreza. It was from NorthUrsula's site. I also found this info.
Looking at the A/F sensor (5th from top), the heaters are on pins D4 & D5 and should give a measurement of 0-1 V according to that table. I measured the voltage with reference to the car's ground (within the engine bay) and got 2.7V w/the car off and 3.2V w/it on...

I did a sanity check and measured the voltage of something else on the table. I took the Rear o2 sensor heater, C13, and got something way out the expected 1V as well. Finally, I measured the Throttle position pwr supply, B9, and got 5V, which is correct. So my sanity check, sort of checked out, but it's not very convincing.

I checked the wire going from the ecu to the sensor plug and it is OK. Until I take the wheel off, I won't be able to check the wire going to the sensor it's self.

I have a feeling, most likely, the wire will be fine and I'll have to put a new sensor in to really find out.

I haven't found any info on how to check the sensor's heating element it's self.

Any ideas?
 

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Its just a heater element. It has a resistance value that could be checked, if you can find it. It receives a duty cycle from the PCM to achieve desired temperature. Given flaky info sources, a new sensor might be the "sane" way to go.
 

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Discussion Starter #11
Just wondering if you think buying a used sensor is a bad idea. Lets say I could get one for half price, say $75, would it be worth it?
 

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with a code like that it's a damn good bet that the problem is in the sensor itself. i would never buy a used sensor when a new one is fairly cheap, and would go OEM over aftermarket cuz you know it'll work.
 
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