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I'm currently getting about 19.6 mpg city in my 97 Mazda Lantis 323 / Protege 1.8L. According to fuelly a car like this should be getting around 26 mpg. The car has about 127k miles on it, and I don't think the O2 sensor has ever been changed, but no error showed up for it when hooked up to a scanner. I just replaced plugs, oil / air filters and oil and the coolant temp sensor ( I was getting about 10 mpg before that ).

Is there any way to tell if changing it would increase my mileage and by how much?

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I doubt you're going to be able to have someone put a number on this. I'm betting you also know that just because no error was thrown, it doesn't mean that the sensor isn't impacting performance.

Lifehacker notes that replacing them could improve mileage "up to 15%." As @BobCross mentioned, "most people wouldn't consider it worth their time to do the comparison. That said, O2 sensors have a finite lifespan."

Also from Bob, "..at 127K miles, the O2 is probably done. Replacing it is usually not a hard DIY job and is certainly cheaper than what some shops would charge."

Paulster's previous note regarding MAP & MAF is still relevant, since they are also involved in mix calculations.

All of that being said, you'll find all kinds of anecdotal evidence that replacing your O2 sensor can improve mileage, but in the end, "it depends"; YMMV (no pun intended.)

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    You're right that you might be able to measure a performance hit between a new and an old O2 sensor. However, most people wouldn't consider it worth their time to do the comparison. That said, O2 sensors have a finite lifespan. You might want to mention that at 127K miles, the O2 is probably done. Replacing it is usually not a hard DIY job and is certainly cheaper than what some shops would charge.... chat.stackexchange.com/transcript/340/2014/3/29 – Bob Cross Oct 30 '14 at 0:12
  • Thanks, I have plenty of thunder already. My suggestion is to add to your existing answer because it helps the 10000 people who ask the exact same question. Most people won't read the comments so an organized complete answer is our only communication path to them. – Bob Cross Oct 31 '14 at 0:11
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    @BobCross Incorporated your comment text -- thanks! – Lynn Crumbling Nov 17 '14 at 18:40
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    I replaced the O2, reset the ecu, and ran some cleaner through the fuel system, then drove for most of a tank on the high way and calculated a 38% increase in kilometers per liter. I'm sure part of that is the highway vs. city driving, so I'll report back after I've driven a tank 95% city but I think there will still be a significant improvement. – Robert S. Barnes Nov 23 '14 at 17:43
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As @LynnCrumbling stated, this would be hard to put a number on, mainly because it depends on too many factors. This is what I can tell you. When O2 sensors get old, they don't necessarily go bad, what they do is get lazy. When a good O2 sensor is doing its thing, if you were to look at the readings from it, the numbers go all over the place, from top of the range to bottom of the range and everywhere in between. As the sensor gets old, this jumping of numbers slows down. This is called getting lazy. The lazier the sensor gets, the worse your gas mileage may become, due mainly to the fact that the computer can only go on the information which is given it. I'm not sure if it would drop as much as you're talking in your case, but it will drop. When I say drop, it's not a drastic thing. It will just get worse and worse over time, until it gets to the point where you say, "I'm currently getting 19.6mpg ..." and they'll do all of this without throwing a code on your computer.

Like I said, I cannot tell you by how much it will increase, but if they are as old as you say, with the mileage you are talking about, I'd say they are way over due. Most manufacturers recommend O2 replacement every 100k miles. The need for changing the O2 sensors is pretty much a given at this point.

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