CEL code P0031 - Where should I start troubleshooting?
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This is a discussion on CEL code P0031 - Where should I start troubleshooting? within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; Hey guys, My first "Check Engine" light came up this morning when I started my 2003 WRX. I hooked up ...

  1. #1
    Registered User negated's Avatar
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    Exclamation CEL code P0031 - Where should I start troubleshooting?

    Hey guys,

    My first "Check Engine" light came up this morning when I started my 2003 WRX. I hooked up the OBD, and got code P0031 which, thanks to YBNormal07's CEL code list (THANKS!), I know is "Front oxygen (A/F) sensor heater circuit low input". Now, what do you think is going on? What should I check or replace?

    Thanks!!

    -S

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  3. #2
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    What year car?

    It sounds like the front 02 sensor might have taken it's last breath. Semi easy to replace located before the UP in the exhaust manifold.
    0-60 faster then your first sexual experience.
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    Registered User negated's Avatar
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    Quote Originally Posted by 302@12psi View Post
    What year car?

    It sounds like the front 02 sensor might have taken it's last breath. Semi easy to replace located before the UP in the exhaust manifold.
    Hey 302,

    It's a (stock) 2003 WRX sedan. Is there a good picture of the sensor/area on the web somewhere so I know what it should all look like together/apart? I've never messed with sensors or their wiring before, so this is all new to me. Thanks for the info!!!! I really appreciate you getting back to me so quickly. Hopefully I can do it myself before the catalytic converter blows...

    -S

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    The only reason I know where is it is because we recently had one apart recently. If it's in the same area as later years it's going to be on the passenger side almost beside the wheel. You will need to remove the up-pipe/exhaust manifold heat sheild (4 screws), skid plate, and probaly the wheel for easy access.

    02 sensors can be had for 109 on the internet...you'll need a special 02 sensor socket also.
    0-60 faster then your first sexual experience.
    "You've come far, and though you're far from the end
    You don't mind where you are, cause you know where you've been"

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    I had the same code recently and I'm also in the process of replacing the front O2 sensor. 302@12psi is dead-on, to access the sensor you will need to remove the front passenger wheel and the small plastic shield attached to the wheel well. Next, you'll need to detach the negative battery terminal and disconnect the sensor cable (located under a cover in the corner of the engine bay (top, passenger side). Next, detach the sensor wire from the block attachment point (follow the cable down). To remove the sensor, you'll have to purchase a special socket. This is where I encountered a big problem. The sensor sits at an awkward angle in the up-pipe and there's not enough room to insert a ratchet into the socket. I tried to remove the sensor using the socket and a wrench and I ended up stripping the sensor hex. Has anyone else had this problem? The sensor is stuck in the up-pipe and yes, I tried to remove it while everything was cold and with a generous application of solvent. What's my recourse at this point? I suppose my next option is to disassemble the upper exhaust, has anyone else attempted this?

    On a related note, for a month prior to sensor failure, I've been using 87 octane fuel. My WRX is an '02 with 50k miles on it. Does the lower octane fuel in any way contribute to premature sensor failure?

    Anyway, Negated, good luck with the removal and keep us updated with your progress.

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    I do not know of any super tricks but there is hardly a situation a good pair of vice grips cant fix. The problem is you might end up stripping the threads in the UP. It's probaly a much safer bet at this time for you to pull the UP (while you are there replace it with a non catted unit).
    0-60 faster then your first sexual experience.
    "You've come far, and though you're far from the end
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    Registered User negated's Avatar
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    FYI, I'm all set. It turned out to be the O2 sensor as expected; got it changed...all set!

    Thanks guys.

    -S

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    I'm jealous!

    I finally replaced the front O2 sensor, what a process that was! The sensor was fused to the manifold and I had to remove the manifold and apply heat, a chisel and tons of torque to finally remove it. After the sensor was replaced, my WRX still throws a CEL and P0031 code. I double checked all of the circuits and everything looks good to me. Giving up, I brought my WRX to a local repair shop, where she's been sitting for two days. The mechanics there are baffled, the suspect that either the ECM is shot or there's a ground fault somewhere in the circuit, which they cannot find. WTF?

    On another note, I just bought this car two months ago (1,000 miles later) from a private party. Would it be right for me to contact the seller to try to recoup the repair expenses ($500 or more if the ECM is shot)? I don't think the seller had any idea about this ground fault and it would have been impossible to hide.

  10. #9
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    Used cars are usually bought as is unless stated otherwise. It's not up to him to front any bill for any repairs.

    Are you sure you didnt mess up the 02 sensor on the reinstall? It's in there at a funny angle.
    0-60 faster then your first sexual experience.
    "You've come far, and though you're far from the end
    You don't mind where you are, cause you know where you've been"

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