Fuel Economy Differences after O2 Sensor

[methodmix]

I recently had a smog test indicating my tailpipe hydrocarbons were much higher than average, and the guy at the shop suggested it could be an O2 sensor going bad, which would cause my engine to run richer than normal. My fuel economy has been relatively stable over the last couple years, although it does seem to be a bit lower (maybe 1-2 mpg) since last year.

I'm averaging about 26-27 mpg on a tank (70% highway driving), with a 2000 Altima GLE (4 spd Auto with O/D). On some long distance trips I can still break the 30-31 mpg mark.

The guy at the shop says it's quite common for the O2 sensor to be degraded without the service engine light coming on (my service engine light is not on), and combined with the labor cost could be a good reason why the O2 sensor seems to be the last item on the list when it comes to vehicle maintenance.

So here's my question. Has anyone noticed a significant MPG improvement after replacing their O2 sensor? When I say significant, I mean at least a 2 MPG improvement.

Thanks.

[nissan cat]

Changing a sensor might help with mileage like you said 1-2mpg is great considering the cost of just a sensor. Over time the metal in the sensor takes a lil' bit longer to send a signal to the pcm. Hydrocarbons means unburned fuel, meaning a possible cat not doing its job. As long as the dtc is not on i'd say buy the sensor, within budget, much better than the price of a cat imo. If it were me, if it aint broke dont fix it, 28mpg? good to me in la, i get the same in my 2.5. For some reason 36mpg at 90mph on my car ,wtf?

[methodmix]

Hey, thanks for the advice! I'm probably not going to replace the O2 sensor until just before my next smog test, just under 2 years from now. How are you figuring 36 mpg at 90mph? Do you have a way to get instantaneous MPG while at that speed? Perhaps you are going slightly downhill, that would be my guess. LA has a lot of hilly areas. Cheers.

[nissan cat]

this model(sl) does have an mpg meter, not intant but it works, 36 at 90 was what i get at times when im out on 138hwy or riverside county when i take a drive. you may be right and again its not intant, im not claiming its all the time. I did some checking on my drive home tonight i got 33.6 at 80mph, could not go faster. My car does have cai, exhaust, and i even throw in synthetic oil and have my tires up to 40psi(51limit cold), so this may contribute. For the most part its how you drive, i go fast but dont race it, good luck!

[robertwilliam03]

Hai,
Once the sensor is hot, a zirconia-type O2 sensor will generate a voltage signal that can range from a few tenths of a volt up to almost a full volt. When there is little unburned oxygen in the exhaust, the sensor usually generates 0.8 to 0.9 volts. The PCM reads this as a “rich” signal, shortens the duration of the fuel injector pulses to reduce fuel delivery, and leans out the fuel mixture.

[methodmix]

Originally Posted by robertwilliam03

Hai,
Once the sensor is hot, a zirconia-type O2 sensor will generate a voltage signal that can range from a few tenths of a volt up to almost a full volt. When there is little unburned oxygen in the exhaust, the sensor usually generates 0.8 to 0.9 volts. The PCM reads this as a “rich” signal, shortens the duration of the fuel injector pulses to reduce fuel delivery, and leans out the fuel mixture.

Thanks for the info! I think what's happening is perhaps a 1 or perhaps a combination of the following:
1) my heater circuit (responsible for heating up the O2 sensor) isn't heating up the O2 sensor quick enough, forcing my vehicle to operate in "open-loop" for most of my morning commute (traveling 9.5 miles),
2) my O2 sensor is going bad due to exposure and contamination, forcing the O2 sensor to read a leaner than normal mixture, which will then force my PCM to provide for a richer than normal mixture

Since the smog testing location was less than 1 mile from my apartment, it's very possible that my vehicle was still in "open-loop" during the smog test, which could very well explain the well above average hydrocarbon readings.

At any rate, I thought I would pass this link along since I think it explains the issues and role of the O2 sensor pretty well:
http://www.aa1car.com/library/2004/us10438.htm

Cheers!