New O2 sensor = better mileage?

[paddlenbike]

Have any of you WITHOUT a check engine light ever replaced an O2 sensor and seen an improvement in fuel economy? Second, would replacing the front sensor ($57) do anything if the rear one were still original?

I have 115k on the odometer and no check engine light; fuel economy is down a bit to 16 city, 19 highway. While working on the 4Runner I lost the O2 gasket, which is $10 vs $57 for a whole new sensor with the gasket, which is what started this whole thing.

[Seanz0rz]

I replaced both and saw a small improvement. Denso from sparkplugs.com

[4x4mike]

My experience was with the air/fuel sensor (I only have one o2 sensor). In researching my issue at the time guys replaced the A/F sensor when they got a cel and mileage took a major dump. I had the light but mileage stayed the same. I replaced the sensor within 25 miles so I never saw the drop. After replacing the sensor my mileage did go up to 25 mpg (loaded highway driving) for about 3 tanks, after that it went back to normal.

In my reading guys got down to 8 and 10 mpg so the sensor needed replacing. I never saw anyone else get an increase. I wouldn't spend an extra $50 for the sensor just for short term gains. I believe the sensors have a 60-75k mile life but most last longer. Maybe buy the kit and shelf the sensor until you need it.

[4x4mike]

Sean, did your increased mileage stick around? Did they drop prior to replacement? Or did they just go back to normal?

[paddlenbike]

Maybe buy the kit and self the sensor until you need it.

That's not a bad idea. I might need it to pass smog next time.

Mine uses 234-4161 front and 234-4154 rear, which are apparently the identical sensor, just different wiring harness lengths. Each are $57 on amazon. For future reference Mike, yours was probably 234-9003 (air fuel ratio sensor) and is available for $107 on amazon.

1999 & 2000 49 State manual V6 4WD - Front: 234-4161; Rear: 234-4154
1999 California manual V6 2WD/4WD 4Runner - Front: 234-9003; Rear: 234-4153

[Seanz0rz]

I think I might be confusing 2 vehicles here.

I think I have 1 o2 on the 4runner. I replaced it and saw mileage return to normal. no Cel, just bad mpg

[YotaFun]

Sean it depends on the year and the emissions equipment.
96-98 Cali or Non Cali had 2 o2 sensors, 1 in front of the cat and 1 after.
99-02 is where it gets fuzzy,
Most are Cali spec and have a A/F ratio Sensor before the cats (yes that is plural there are two) and an o2 sensor after the cats.
I guess according to what Paddlenbike came up with the 99-00 non Cali spec uses two o2 Sensors like the 96-98.

FWIW back eons ago when I got my 4Runner, my first CEL experience was with the rear o2 Sensor, and while everyone believe that doesn't contribute to mileage, in my case it did, and dramatically as well as performance.
I was seeing a major decrease of about 5-6 miles per gallon as well as the truck running very sluggish. I would clear the CEL and the truck and mileage would be fine till it went through its cycle.
At the time I was young an stupid so I just got the cheapest one I could afford and threw it in (no name brand) CEL went away and mileage seemed to be normal.

Eventually through the years my front o2 sensor went and I replaced that with a Denso, and the CEL hasn't been back since.

However my mileage has been considerably more crappy by the day and I believe it is due to that rear o2 sensor, so in the next few week I plan to replace that with a stock Toyota one and see if that doesn't help me out a little bit

[paddlenbike]

All 4Runners have two O2 sensors, one before and one after the cat, regardless of make/model/year/emissons package.

96-98 are all pretty similar
99-00 get confusing due to CA emissions package, many of which use air fuel sensors rather than oxygen sensors.
01-02 are pretty much like the 99-00 CA models.

http://www.urdusa.com/More-Products-..._13/index.html
While you do see varying part numbers, most 96-00 non-CA trucks use the same sensor, they just have different wiring harness lengths and thus the difference in part number.

[Seanz0rz]

again, it has been a while since ive done it. if there were 2, i replaced both. i suppose i am mistaken with o2 sensors and cat. i only have 1 cat.

[paddlenbike]

haha, it's okay Sean, it is ridiculous how many variations there are.