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Discussion Starter #1
A couple months ago, I helped my daughter buy her first car and I steered her to a 2012 SE 5-speed with just under 115k miles. I have a 2012 SEL automatic and I like it but I would rather have the manual transmission if I were to do it all over again.

So, other than the transmission and a few options, it's the same car and so far, I've been the one putting most of the miles on the manual as she's been learning to drive a stick and I've had some minor repairs to take care of. At any rate, I noticed that her gas mileage was a bit less than what I've been getting in mixed driving - just under 29mpg vs 32mpg. I thought that this was in part due to the transmission differences.

Then about a week ago, the engine threw a code indicative of the upstream O2 Sensor. So I replaced it with an OEM sensor and did some driving today after resetting the fuel economy setting. Over the 25 mile drive in similar conditions to my typical commute, I averaged 37mpg. I'll keep an eye on this but I think that this abrupt improvement can be attributed to a properly functioning O2 sensor. In doing some research after the fact I found that the typical service life of the sensors is 100K miles and that a poorly functioning sensor can decrease fuel economy up to 40 percent.
 

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That's something they would say at AutoZone on a billboard in the store. Your O2 sensor was failing and that's why you got it code it was on its last leg. I'm glad your gas mileage went up but there's just no need to replace oxygen sensors just for the hell of it.

Sent from my LG-LS997 using Tapatalk
 

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Discussion Starter #3 (Edited)
That's something they would say at AutoZone on a billboard in the store. Your O2 sensor was failing and that's why you got it code it was on its last leg. I'm glad your gas mileage went up but there's just no need to replace oxygen sensors just for the hell of it.

Sent from my LG-LS997 using FF Mobile
Believe me when I tell you that prior to this, I had the exact same mindset but I drove it for 1000 miles without seeing a CEL but was getting poor mileage so you can have a poorly operating sensor without it throwing a code. I should also add that the CEL light had gone off again prior to me replacing the sensor so as far as the ECU was concerned, it was borderline working.

I will say that there are other likely suspects when the engine seems to be running well but mileage is poor like dirty injectors or a sticking EGR valve.

So, while I'm not one to do proactive preventative maintenance by replacing parts when they reach their expected service life, this is a case where you might not want to wait for a CEL if you have other indicative symptoms. If my mileage improvement holds up, it will pay for itself in under 8 tanks of fuel.

If you're interested in additional information on how O2 sensors work and how they can fail prior to throwing a code, see this.
 

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O2 sensor lifespan varies too much to replace for preventative maintenance.

If anything, replace the spark plugs, those are better indicators and much easier and cheaper to do. Cylinder 1 will always take a beating on these.
 

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O2 sensor lifespan varies too much to replace for preventative maintenance.

If anything, replace the spark plugs, those are better indicators and much easier and cheaper to do. Cylinder 1 will always take a beating on these.
For my own education, why does cylinder 1 especially take a beating?
 

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Discussion Starter #6
O2 sensor lifespan varies too much to replace for preventative maintenance.

If anything, replace the spark plugs, those are better indicators and much easier and cheaper to do. Cylinder 1 will always take a beating on these.
My point was that there are things other than the CEL that point to a bad sensor. If you have none of these symptoms then I totally agree that there is no reason to replace it as a preventive measure.
 

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O2 sensors are a wear item, same as plugs....A good percentage of ppl don't swap then out till they have issues.
 

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I for one do believe that the upstream O2 will show sometimes a need for new by simply the mileage dropping off some, the car will continue to drive fine as the drivers do not notice unless they stay on top of mileage numbers. Most sensors do not die at once fast unless somebody drives car flooding or missing badly, other than that they tend to die slowly by just slowing down the switching they do little by little.
 
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