Replace O2 sensor? (and how?)

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Old 01-07-21, 12:09 PM
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Replace O2 sensor? (and how?)

The license plate renewal on my 2001 Subaru comes every December, and where I live we have emissions testing. Every year since I've owned this car, when the autumn weather first turns cold and wet, I get a (O2 sensor) CEL on my Subaru. I do all the auto maintenance I'm equipped to (both tool-wise and skill-wise) so the first time this happened I had hoped to change the sensor myself but I couldn't manage to break it loose. But to my amazement, the CEL light went out in time for me to get to the testing station and pass emissions in time to renew my.

I've also discovered since then that I can do my emissions testing as much as 90 days before renewal is due. So now I make a point to get my emissions testing done as soon as I can remember in October, which is usually before it gets cold and wet.

Problem solved except that as the years go by and the miles add up, my gas mileage also decays slightly. Now I'm getting 2 or 3 mpg less than when I first bought it (~80k miles ago), and I can't help but wonder if I might get some of that back with a new O2 sensor.

I should note here that overall it runs well enough but idle can on occasion be a bit erratic and a handful of times it has stumbled when I got heavy-footed from a standing-start. But still, for a car with 220k on the clock, I am nowhere near thinking it's ready for the scrap heap.

All that to get to this.

Question #1:
If it were you, would you let it ride as long as the car passed emissions and fuel economy wasn't truly horrible, or would you bite the bullet and replace the sensor? Or something else?

Question #2:
How do you deal with a frozen O2 sensor? I should note I didn't get especially aggressive when I tried to remove this one. I'd left is soaking overnight in PB Blaster but I only used an open-end wrench, not one of those sockets with the cut-out to avoid the sensor wire. And although I twisted as hard as I could, I didn't try putting a cheater bar on the wrench for fear of twisting the sensor in half (can that even happen?) or rounding off the hex surfaces.

So what 'tricks' do the pros use for changing stubborn sensors? If I get the socket that's custom-made for the job, is there a risk of destroying the sensor before it breaks free (after which I'm facing an even bigger repair bill than just replacing the sensor)?

I ask this now because I've got most of a year before I'm due testing again, so I've got a log of cushion if I were to try DIY-ing it now.

And for obvious reasons I'd sooner not go ahead and cut the sensor wires and just go for it with a 'regular' socket until I know I can get the cussed thing out.

So any advice, comments and jeers are welcome.
 
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Old 01-07-21, 12:18 PM
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If you want jeers, first off, i laughed when I read that you were trying to do it with an open end wrench. You need the cutaway O2 socket and you need a cheater bar. I believe I got mine from Harbor Freight, ($8) it worked fine. You just need more leverage to break it loose. And yes, its always possible you could break it. But even if it breaks there is no reason you could not continue to remove it unless you are just a weakling. LOL

But I expect that nothing will change if you change the sensor. Typically when you get a warning light it is something other than the O2 sensor that caused the light to go on in the first place. Changing an O2 sensor can affect gas mileage, but there is no guarantee it will... because it could be that there is nothing wrong with it in the first place.
 
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Old 01-10-21, 08:09 AM
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Thank you, XSleeper.

For my general mechanical edjumication, and never having seen a sensor that had been broken off, I'm curious to know how (mechanically) challenging it likely would be to remove what's left behind. I'm imagining it's something that would have to be drilled out. Or is it less challenging than that?
 
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Old 01-10-21, 08:16 AM
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Imagine a broken spark plug. Without the insulator you could then use a normal socket on it. Breaking an O2 sensor is not an obstacle. Before I had the socket I used a grinder to cut them in half so that a normal deep well socket could be used.
 
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Old 01-10-21, 10:25 AM
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All filed away for future use. Thanks again.

 
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Old 01-11-21, 02:09 PM
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So the most obvious question, when you get the light have you pulled the codes from OBD2 to confirm it's an 02 sensor or was that done at some point in the past?

And like any type of exhaust work getting that off the first time is going to be a PITA, all the corrosion!
 
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Old 01-16-21, 07:26 AM
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So the most obvious question, when you get the light have you pulled the codes from OBD2 to confirm it's an 02 sensor or was that done at some point in the past?
Strictly speaking, ... no. All I get on my code reader is P0420, which is cat inefficiency. I haven't paid a professional mechanic to give me a more definitive diagnosis.
 
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Old 01-16-21, 07:35 AM
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Its often a waste of money to just replace parts hoping you will fix a code... which is why I mentioned that there could be nothing wrong with the sensor. You don't know if or which sensor triggered it (upstream or downstream) and that code could be due to other things. A failed catalytic converter. Vacuum leaks. Circuit problems, such as damaged wiring and loose connections. An exhaust leak.

If you have live data on your reader, you can often look at your O2 sensors in wave form and verify their voltage levels under various driving conditions... driving, accelerating, decelerating and at idle. This is just additional diagnosing info that gives clues as to the O2 performance.

I once determined that what I thought was a bad O2 sensor (seemed dead at idle) was actually a manifold vacuum (gasket) leak that only was affecting the O2 sensor at idle, when airflow was very low.

In the past, I've also taken cars to the mechanic, was told O2 was bad and that they replaced it, only to have the same code occur again soon thereafter.

 
 

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