o2 sensor code and horrible mileage

[mastershake]

hey guys, my 03 alero 3.4 is getting about 11 miles to the gallon right now (about 160 miles to the tank) and i drive mostly highway. the service engine soon light is on and i took it to autozone and they said it was the pre-cat oxygen sensor (i didnt ask for the exact code :/). so i replaced the oxygen sensor and reset the computer, and the light was gone for 2 days and then came back. went back to autozone and they said it was still the oxygen sensor. there are really no other symptoms besides bad gas mileage... it starts fine, idles perfectly, has enough power, and theres a normal amount of exhaust coming out the pipe so i dont think the cat converter is clogged... does anyone have any ideas?

by the way its got about 203k miles on it

and the owners before me drove on a dirt road. could it be a dirty MAF sensor?

[guiguilandry]

check your plugs and wire, a dirty maf could be the culprit too, while you're at it check your air filter ... is the ses light flashing when you are driving it or is it only lit up ?

[mastershake]

just changed the air filter a thousand miles ago. the ses light is solid not flashing. spark plugs could be a possibility but could they make my mileage THAT bad?? and would they be giving me the code?

[guiguilandry]

I was thinking that there was a possibility of misfire, but if its not flashing than it's not that... did you get your iac unpluged by mistake when changine the air filter... and wich o2 sensor is giving you a the code post cat or the pre cat

[mastershake]

the ses light was on before i changed the air filter. and its the pre cat thats giving the code.

[guiguilandry]

You can try to clean the maf and look at the plugs too.. that's you're next step

[colonel6632]

if it's been warm and cold where you live, maybe you have water in your gas. that caused my primary 02 sensor to not read right and throw a check engine light

[mastershake]

i dont think that could be it either. the ses light has been on long before i even bought the car, and ive filled up many times since (i got the car last november)

[mastershake]

ok, sorry for the double post, but i went to check the MAF to see if its dirty, and cant even get the thing off. i undid the clamps on the hoses going in and out of it, but the hoses are still STUCK on it.. how the hell do i get this thing off??

[BoarderTy]

This same problem was happening on my alero (2000 2.4L Coupe 160,xxx). but mine happened after the fuel pum,p was replaced. after dealing with it for 3 weeks and talking to a shop about getting my car looked at...the day it was supposed to get looked at at the shop i started seeing a difference in my fuel usage. it was SIPPING the gas instead of Guzzling! and i had the shop clear the code (Po440) or something along those lines which is just an evap code and i had been running with that for good part of the last year and as soon as they cleared it a day later if came back up and it runs fine...

[Lucalare]

i dont think that could be it either. the ses light has been on long before i even bought the car, and ive filled up many times since (i got the car last november)

You bought a car that had an SES light on, are you stupid or something?

[mastershake]

You bought a car that had an SES light on, are you stupid or something?

no, im not stupid. its my first car. my mom bought it for $900 from her friend at work, who only used it for work, so the 200k miles that are on it are all highway miles. the dude who sold it to us told us the light was just a bad o2 sensor and it would only down the mileage by 1-2 mpg's... i did not know it would be getting 11 miles to the gallon lol

[zzyzzx]

If I understand you correctly, you replaced one O2 sensor when there is most likely more than one on the car. At 203K miles, the original O2 sensor(s) should be long gone.

[mastershake]

If I understand you correctly, you replaced one O2 sensor when there is most likely more than one on the car. At 203K miles, the original O2 sensor(s) should be long gone.

correct. i replaced the one thats before the cat converter which is the one that actually regulates the fuel/air ratio.

i dont really think the post cat sensor has anything to do with it. yes, it probably should be changed soon but thats not my top priority right now :p

[Ttop191]

all the post cat sensor does is check to make sure the cat is actually doing it's job

[mastershake]

all the post cat sensor does is check to make sure the cat is actually doing it's job

yup so i know thats not contributing to it

[Gr1m]

the post cat sensor will throw a code if it does not read the numbers the PCM wants.

[mastershake]

the post cat sensor will throw a code if it does not read the numbers the PCM wants.

right, but its the pre-cat sensor thats giving the code

[clutch1]

Time to look at fuel trims to see if it's adding a ton of fuel because of a vac leak, or taking away fuel because an injector's leaking or something.

Do that.

[widbyj]

Time to look at fuel trims to see if it's adding a ton of fuel because of a vac leak, or taking away fuel because an injector's leaking or something.

Do that.

I really doubt this person has a Tech2 or HPTuner to monitor his fuel trims himself.

[mastershake]

I really doubt this person has a Tech2 or HPTuner to monitor his fuel trims himself.

LOL this is true.

[Cliff8928]

Actually, even the lesser expensive scanners from autozone, etc.. can view the fuel trim data.

[mastershake]

hey guys so i cleaned the MAF today along with the throttle body, started her up and NO CODE!!! im hoping the gas mileage is severely improved :)