How to Replace Upstream Oxygen Sensor (CEL P0031)

If you prefer to watch a more descriptive video check this out:

Otherwise here are the cole's notes.

I got a check engine light with code P0031 - Heater Control Circuit Low (Bank 1 Sensor 1) which sent me on a hunt for info and how to fix it. Here's what I've pulled together and should help anyone else with the same issue. Our cars have a sensor that is downstream of the catalytic converter and one that is upstream, also known as an air/fuel ratio sensor. Bank 1 is located on the passenger side of the car. Therefore Bank 1 Sensor 1 is going to be located pre-cat on the passenger side of the car. Heater Control Circuit Low refers to a failure of the heater circuit that heats the oxygen sensor to help get it up to operating temperature when cold. Your best bet with this failure is to replace the sensor with a new one. I ordered mine off of Rock Auto for $116 CAD part number 2349120.

Start by taking off the front passenger side wheel and put the car on a jackstand.

http://legacygt.com/forums/attachment.php?attachmentid=211595&stc=1&d=1435295420

Remove the push-pin plastic rivet circled here and pull the skid plate down to expose the O2 sensor.

http://legacygt.com/forums/attachment.php?attachmentid=211596&stc=1&d=1435295420

Head into the engine bay and unplug the old sensor from the harness. The plug is near the rear of the engine bay near the strut tower.

http://legacygt.com/forums/attachment.php?attachmentid=211597&stc=1&d=1435295420

Unscrew the sensor using an O2 sensor attachment for your socket wrench which can be bought for around $15 and is worth it in this case and then unplug these connectors from the old sensor cable in the engine bay.

http://legacygt.com/forums/attachment.php?attachmentid=211598&stc=1&d=1435295420

Here's what the new sensor looks like. It's blue as opposed to yellow for some reason but the sensor part looks right.

http://legacygt.com/forums/attachment.php?attachmentid=211599&stc=1&d=1435295420

Now connect the new sensor in the engine bay and feed it down through where the old sensor was. Screw the sensor into the socket using your wrench and then clip the connectors back into their spots in the engine bay. Screw the sensor in first or you'll get a wire that looks like mine does in this shot - curly

http://legacygt.com/forums/attachment.php?attachmentid=211600&stc=1&d=1435295420

Button her all back up the way you found 'er and fire up the engine. If your CEL is still staring at you reset your ECU. Everything should be fixed now. If it isn't you've either done something wrong or you didn't really have a malfunctioning sensor and something is probably seriously wrong. Otherwise you're done!

One tip I give for doing this job is to put a screw type hose clamp around the socket after its on the old sensor to keep it from spreading and rounding off the hex on the old sensor.

Also spray the old sensor with PB Blaster and let it soak in for a few minutes.

I also put a small dab of anti-seize compound on the threads of the new sensor.

Your info was much appreciated. Shogun, I didn't remove the wheel or lift the car, and I'm not a small guy. Access was great. Max, I compared two sockets at Harbor Freight. The long socket has a wide gap in the wall to allow for the wires but the length looked like it could be a problem in restricted areas. The "offset" socket has a more narrow gap and probably less flex. The only downside is as the socket rotates the offset part you plug into your wrench interferes with continuing the ratcheting. You need to remove the socket and start over after about 180 degrees of rotation.

I looked at the downstream sensor today but did not have the car (2005 LGT wagon) on a lift. The wiring from the sensor disappears into the body/chassis and my only manual is a Haynes (useless). Any tips for removal of the downstream sensor? I'm hoping for an easy removal absent any disassembly of other parts.

Anyone? I'm going to put the car on jackstands later this week, but are there any surprises for me?

Look up how the WRX guys do it on youtube and extrapolate for your car it's pretty much the same thing from what I can see.

Look up how the WRX guys do it on youtube and extrapolate for your car it's pretty much the same thing from what I can see.

Do you have a link? I tried some videos and the best I could do for the rear sensor was some kid struggling with the sensor (he didn't use solvent to break the rust) and the comments session had everyone ragging on him for not using jackstands (he did use at least one). Nothing specific on where the wiring harness led to.

Do you have a link? I tried some videos and the best I could do for the rear sensor was some kid struggling with the sensor (he didn't use solvent to break the rust) and the comments session had everyone ragging on him for not using jackstands (he did use at least one). Nothing specific on where the wiring harness led to.

I changed out my downpipe for a CNT 3" catted one yesterday and to do so I had to remove the rear O2 sensor. Well it turns out my sensor was pretty seized and I am too strong for my downpipe . My sensor came right out of the downpipe, still attached to the bung it sits in. Safe to say I have a hole in my stock downpipe and I'm ordering a new O2 sensor right now. Anyway when you do this use lots of penetrating fluid.

So I tried to take a couple of pictures but it was difficult. Anyway when you get under the car, the sensor is very obvious where it attaches to the downpipe. Now you undo it there and follow it along the bottom of the car to the harness which is hanging down from where the downpipe goes up to the turbo. Disconnect it there and feed the new one through exactly the same path and there you go, problem solved. Should take about 15 min. Anyway I will try to upload the pictures soon. I will need to get under there again when my sensor arrives anyway.

Do you have a link? I tried some videos and the best I could do for the rear sensor was some kid struggling with the sensor (he didn't use solvent to break the rust) and the comments session had everyone ragging on him for not using jackstands (he did use at least one). Nothing specific on where the wiring harness led to.

Here's a couple pics of what you're looking for. The first is the sensor in the downpipe. The second is where it disappears into the car. That's a shot from the bottom looking up so look near the downpipe and you'll see the harness you need to disconnect.

Bumping to say thanks for the write-up. I used these instructions and this socket and it worked like a charm.

Symptoms before were awful gas mileage, CEL with code P0031, and seemingly a bit of power loss. With the new sensor installed and codes cleared, everything is back to normal.