I'm looking to change my o2 sensor or my air-fuel ratio sensor. The one that determines my gas mileage. Right now I'm getting like 9 mpg city and I'm running around 80k miles so I think changing it will greatly benefit my mileage.

I'm lookign to take it to a shop to do it, but what do I tell them to change specifcally? I know there are multiple oxygen sensors on the 4runner. Also how much should I be expecting to pay?

anyone?

The two upstream sensors, before the Cat, are the ones responsible for the fuel management. The downstream ones are only there to indicate when the catalyst is spent.

Ok, I wanted to rule out a couple of the obvious points first:

First, you live in SF, and if you're driving up and down those hilly streets, your MPG will suffer. Plus, judging by your sound system, I'm guessing you don't have the most conservative gas peddle (). Also, how is the traffic? A lot of idle time? With going up hills, stomping on the gas peddle and hitting a lot of traffic, I can see 8-9mpg. Lastly, how are you calculating your MPG?

I don't think you need to change out your O2 sensor if you're at 80k, unless you're getting a CEL.

I didnt have to replace my front A/F ration sensor until i hit 117K miles. But I didn't know it was going out until the Check Engine light went off. To change the sensor is a 20 minute job. Living in SF you probably don't have any rust so that makes life easier.

have you changed your timing belt recently? they are supposed to be changed ever 60k according to toyota. you may not need an o2 sensor or a/f sensor yet, i just replaced my a/f at 120k and my o2 at 90k try a basic tune up and see how that helps

plugs, wires, clean maf/throttle body/ air filter, change oil and filter....

It sounds like you're pretty sure this is whats wrong. You would want to change the two front o2 sensors and i would recommend changing them yourself its as easy as unbolt/unplug and rebolt/replug its a relatively easy task as long as nothing is seized (i usually just heat up the bolts with a torch). I believe the two sensors would be around $250 but that may be off.

Yes I am driving only in the city with a lot of stop signs and stop lights. I don't drive slow but I'm not a speed maniac. I don't really have too big of a lead foot. I calculate my mpg by fill it up to the top, recording how many miles I get, and than filling it up to the top and dividing miles by gallons.

When I first got the car I was at like 11 mpg, now I'm at a constant 9 mpg and my driving style has not changed. So it is getting worst.

Also, I just went on a long road trip on all highway driving to Los Angeles. I got a little less than 250 miles on a tank. I've noticed almost everyone else gets at least 300+ when they drive on all highway.

And I've gotten a tuneup in the past and it's really done nothing to help my MPG.

There is no real reason to change an O2 sensor unless it is bad. The vehicle will tell you if it's bad. If it doesn't tell you (by way of MIL) they only to tell is to look at your air/fuel ratio. O2 sensors can throw that off, which can also cause further damage to the emissions system. But if there is not a light it should be fine. As stated previously, the sensors can be tough to remove so you may need a torch to heat up the metal. I guess it doesn't make sense to me to replace a perfectly good part without the vehicle saying it's not working properly. Your car is also getting older by the day so that can be a factor.

If that was mine I would clean the MAF (mass air flow sensor)first. Just spray it down with electrical cleaner. I do this every 30K miles.

just the O2 sensor for mine at Autozone is $140

Just to clear it up, I'm trying to change the sensor that determines how much gas is being spent, I believe its called the air-fuel ratio sensor but some people call it the oxygen sensor also.

I don't know, the 9 mpg is very low in my opinion. Shouldn't I at least be getting like 14 or so?

I've had very minimal engine work, I'm not sure if I should do this on my own or let someone that knows what hes doing do it. I don't want to run into a problem which would end up costing myself more money.

Anymore opinions on whether I should change it or not? If 9 mpg given my conditions is normal?

Oh, I also ran across this while doing some research.

http://www.4x4wire.com/toyota/4Runner/tech/O2_Sensor/

OK, I give in- REPLACE THE DAMN THING ALREADY!!

Haha I'm not trying to be stubborn, but I'm just really unsure that 9 MPG is normal for a toyota 4runner thats only 7 years old. I mean it's not that big of a car.

I mean if I can get enough people telling me that its perfectly normal, than I'll give in, but so far I haven't really met anyone thats gotten less than 12 mpg so far.