O2 sensor replacement gives better gas mileage

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Just replaced my upstream O2 sensors, due to poor fuel mileage from rebuilt engine. No check engine codes, just wanted to see if mileage would improve. On the old O2 sensors, I averaged 16.7 MPG per tank of fuel. I’m now averaging 19.6 MPG per tank with the new sensors. I drive the same route every day, so conditions should be the same. Old sensors had 259,000 miles on them, so well beyond their service life. I’d say if your high miles rig is suffering poor fuel mileage, then replace those upstream O2 sensors.
 

lfhoward

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Thanks for this. Am at 191,500 miles and will keep this in mind if my mpg drops. How quickly did yours change— was it a sudden dropoff in mpg or a slow process?
 

klc

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Just replaced my upstream O2 sensors, due to poor fuel mileage from rebuilt engine. No check engine codes, just wanted to see if mileage would improve. On the old O2 sensors, I averaged 16.7 MPG per tank of fuel. I’m now averaging 19.6 MPG per tank with the new sensors. I drive the same route every day, so conditions should be the same. Old sensors had 259,000 miles on them, so well beyond their service life. I’d say if your high miles rig is suffering poor fuel mileage, then replace those upstream O2 sensors.
I should probably do this eventually as well. Thanks for the tip.
 

DadOSix

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Just replaced my upstream O2 sensors, due to poor fuel mileage from rebuilt engine. No check engine codes, just wanted to see if mileage would improve. On the old O2 sensors, I averaged 16.7 MPG per tank of fuel. I’m now averaging 19.6 MPG per tank with the new sensors. I drive the same route every day, so conditions should be the same. Old sensors had 259,000 miles on them, so well beyond their service life. I’d say if your high miles rig is suffering poor fuel mileage, then replace those upstream O2 sensors.
Did you use the oem sensors? My 06 is 182k and i'd sure give this a try
 

Mshack

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Just replaced my upstream O2 sensors, due to poor fuel mileage from rebuilt engine. No check engine codes, just wanted to see if mileage would improve. On the old O2 sensors, I averaged 16.7 MPG per tank of fuel. I’m now averaging 19.6 MPG per tank with the new sensors. I drive the same route every day, so conditions should be the same. Old sensors had 259,000 miles on them, so well beyond their service life. I’d say if your high miles rig is suffering poor fuel mileage, then replace those upstream O2 sensors.
Now I want to change all 4!
Looked at them last night after my posting reply on my 2 days of driving my mail route.
The whole exhaust system looks OG, up to the muffler.
So could be a huge part of my fuel mileage issue. Will wait for a good scan gun diagnosis.
 
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My old sensors just declined gradually, so I really never noticed the drop in fuel mileage. The new sensors made an immediate difference, and with gas topping $4/ gallon these days, it would be cost justified. I bought a set of NTK sensors off of eBay for around $64. Look just like OEM sensors.
 

Robochop

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Just replaced my upstream O2 sensors, due to poor fuel mileage from rebuilt engine. No check engine codes, just wanted to see if mileage would improve. On the old O2 sensors, I averaged 16.7 MPG per tank of fuel. I’m now averaging 19.6 MPG per tank with the new sensors. I drive the same route every day, so conditions should be the same. Old sensors had 259,000 miles on them, so well beyond their service life. I’d say if your high miles rig is suffering poor fuel mileage, then replace those upstream O2 sensors.
What kind of driving is this? Highway or City? I can't get more than 15 combined back and forth to work. Straight highway, long trip mileage is much better tho!
 

Ksat

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Now I want to change all 4!

The rears may not have any effect on the fuel mileage, fyi. In many cars they're just used to determine how well the cats are functioning. I've heard some vehicles may use them additionally to adjust long term fuel trims, but I'm not sure if that's the case on the libs.
 

Willie

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Just replaced my upstream O2 sensors, due to poor fuel mileage from rebuilt engine. No check engine codes, just wanted to see if mileage would improve. On the old O2 sensors, I averaged 16.7 MPG per tank of fuel. I’m now averaging 19.6 MPG per tank with the new sensors. I drive the same route every day, so conditions should be the same. Old sensors had 259,000 miles on them, so well beyond their service life. I’d say if your high miles rig is suffering poor fuel mileage, then replace those upstream O2 sensors.
Thanks
 
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My driving is mixed backroads and city driving average 45-55 mph with very little interstate, for which Im getting 19.6 mpg now. I’m getting 21 MPG on the interstate @ 70 mph, but wind and traffic affect It a lot. Have not had a chance to burn a tank of fuel at highway speed yet, so I can only use the average economy data on the trip computer for highway cruising.

I replaced my downstream O2 sensors when I installed the rebuilt engine to correct a faulty O2 heater circuit error code. This corrected the check engine code, but did nothing to improve fuel economy. I only got better fuel economy after replacing the upstream ones
 
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