Quickie 4.153: Replacing My Forward O2 Sensor

As mentioned at the end of my previous quickie I had to replace the forward O2 sensor on the Saabaru so that I could pass emissions and renew my plate stickers. A friend of mine, the same friend from this post, MrCool did most of the work. Not because I couldn’t, but because I’m a pussy and didn’t want to touch the hot exhaust pipes.

Little back story to start us off, I had my car up on jack stands about two weeks ago and was trying to remove the rear O2 sensor from my downpipe and just could not get it to budge. I suspect part of this is due to the fact that I hate being under 3,400 lbs of metal that could crush me, and while I know the jack stands can take that weight (each), I worry about my placement and movement.  As such I was not really giving my full effort for fear the car would come falling down on me if I did (lol, because I’m that strong). So I messaged around getting ideas of how to tackle the problem and MrCool responded with basically, “I’ll come by and do it.” As I awaited his arrival I decided to use this time to check my work. So I plugged in my cheap Chinese bluetooth OBDII reader and paired my Nexus 4 to it, then used an app called Torque, I read the fault codes off once again. As it would turn out, the rear O2 sensor was not failing as per the cars own diagnostics, the forward O2 sensor was. Oops!


I was so sure that it was the one in the downpipe, and I think I know why I thought this initially. Though this is just some speculation on my part because I have little idea how this really all works. I think the rear O2 sensors readings are compared with the forward o2 to make sure the catalytic converter is doing it’s job. When the forward sensor started to fail, the measurements were off for the comparison test and my check engine light came on for bank 2 (rear sensor). However, once the forward sensor failed completely it showed bank 1 (forward sensor) as failing. However, just as likely (possibly more so), I just misremembered.


Never the less, I double checked both sensors myself to be really sure. I measured the current each was receiving to be sure the problem wasn’t elsewhere, and I measured the resistance, to be sure the sensor was functioning correctly. As it would turn out, the forward O2 sensor gave a reading of no reading/open, so clearly it was not functioning properly.

MrCool then arrived and I broke the news to him, that the O2 sensor I’d been trying to remove was not the one I needed to remove, and perhaps his help might not be required after all. But since he was here anyway, we got to work removing the forward O2 sensor so we could take it to Subaru and get a new one. Why it didn’t occur to us at any point that removing it before we had a new one wasn’t needed, because they could look up the part number in their system, they didn’t need to look at it. Oh right, because I wanted to inspect the wire for damage first.


Anyway, MrCool and I then went to Subaru to buy a replacement sensor. Gave them all the information, and the guy looked it up on his computer, told us there was no stock as they don’t get many 2.0L WRX’s these days, which makes sense, they’ve been 2.5L for a while now. Then he tells us it’ll be $417. Fuck. That. But I don’t know of a better price, so I order it anyway. Once MrCool and I returned back to my house and he called up a shop he knew and found me a sensor for $170. I quickly cancelled with Subaru and accepted this new much cheaper order. Perhaps had I shopped around I’d have found a better price, but as I’d already ‘saved’ $247 I felt pretty good.


And now we’re caught up and I can explain why I didn’t want to touch hot exhaust pipes! Since we weren’t going to get the sensor that day and the shop MrCool knew was in Toronto, we put my car back together and decided to meet at a later date to put the new one in. Since I’d have to drive the car anyway to get the sensor data all logged so I could pass emissions we decided I should drive to Toronto, so that the drive home would be enough to get all the data required. This meant the car would have been driven 1.5 hours, on a hot day, through traffic, by the time it got to MrCool’s place. Unfortunately as cool as MrCool might be, he couldn’t cool the exhaust and turbo down enough for me to touch it, so he had to do the work. I placed some jack stands, and took my wheel off, but that’s about it.

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To install the sensor in my car we needed to remove the front right tyre, the plastic clips that hold the plastic wheel well in place, and the plastic skirt that sits under the engine. Nothing too difficult there, but the hotness comes in to play in the location of the sensor, where the wire has to travel, and where it plugs in. The forward sensor is in the uppipe to the turbo, which is pretty much where the hottest exhaust gases will be, thus the pipe will be extremely hot. Next, the wire follows that hot uppipe to my turbo, which is also quite hot, where you’ll find a plastic harness to hold a bunch of wires in place, which also happens to be right next to another hot object, and then finally to the plastic connector where the sensor connects to the car.

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Simple job! I’m glad I didn’t pay someone to do it, well ok, I bought MrCool lunch, I’m glad I didn’t pay someone who’d have cost more money!