rayrobot

So recently, i've noticed that i've been getting really bad mpg, i got 13-14 range in the last two months. In the last two months I have basically been going 80% local rather than the 90% highway when I went to college. And even going 90% highway, i would get around 18-20 mpg. Since schools out, been chilling around the city and I'm receiving terrible MPG (13-14).

I changed my o2 sensors around 1 month ago. Only changed both upstream sensors since after looking through the forums, those sensors are the ones that calculate air/fuel ratio, if i'm correct. My maxima is a 1999 cali spec if that would provide any clue on this mystery. I replaced it with an ebay item :

http://www.ebay.com/itm/SMP-SG285-OXYGEN-SENSOR-FOR-NISSAN-MAXIMA-1995-1999-/380637071482?ViewItem=&vxp=mtr&item=380637071482&nma=true&si=mYzdMwFYKur2wIrl2fJt2e43dgA%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557
1. Has anyone had any experiences with ebay o2 sensors?
2. How can you tell if an o2 sensor is fully working correctly?
3. If the sensor is bad, where would i get a reliable sensor for the cheapest?
4. Do i go oem for the o2 sensor? If so, whats the part number and brand?
5. Any other clues on why my MPG is horrible? (Maintenance is kept up, oil change, air filter, oil filter, spark plugs, might be forgetting alot of other stuff, can't remember off the top of my head)

& i also got a code RIGHT BEFORE i was about to go into the shop for an inspection. Disappointingly i had to drive back home =(.

The code was p0171, engine running too lean apparently.

So i resetted my ecu with my ob2 reader and did the run to make all monitors okay. All my moniters are ready now and i'm gonna go get my inspection done tomorrow. Hopefully i'll pass. Is there anything i should be checking for before i go for inspection?

maxgtr2000

iTrader: (29)

As long as there are no codes and systems are ready you should be good for inspection. Gas mileage calculations rely on quite a few factors. O2 sensor voltage can be checked with an obd2 reader, there are videos on youtube of what the voltages should look like. The switch from more highway to local has an effect on mileage because there is more stop and go. Highway mileage is usually higher because the vehicle keeps momentum, stop and go is what kills mileage.

Other factors include improper tire pressure, cheap gas, lower octane gas, vacuum leaks, bad/old spark plugs, dirty oil, improper oil viscosity, dead or partially working knock sensor, and dirty air filter.

rayrobot

Okay so i passed my inspection today, yay. How much was your MPG for city or highway?

But i'm still very confused on the o2 sensor voltage? Doesn't that vary on the throttle and when you are actually driving? Is the voltage the only way to check if the sensor is working correctly?

Has anyone tried the ebay o2 sensor that i linked above?

As for other factors:
-always used premium gas
-already done spark plugs
-pretty sure knock sensor is working b/c not throwing any codes
-i used 5w30 oil
-air filter is clean
-tire pressure is at 34 psi on 15inch gle rims
-vacuum leak? how do i check that?