Very poor gas mileage + decreased power = bad oxygen sensor?

[AllanS]

Lately I've been getting around 14mpg (down from a previous average of 19) and have noticed a decreased power output. Would it be a safe first assumption that the oxygen sensor might need to be replaced? The check engine light came on briefly once a couple weeks ago, and I havn't done a stomp test yet to see if there's a code stored in the system.

Thanks!

AllanS
91' 535iM

[rickm]

My stomp test came out clear, I replaced my O2 sensor anyway. The car is running better, mpg is up a bit, even when it's cold (getting 23mpg on the highway in the winter vs 28 in the warmer months).

[AllanS]

I just bought one... the car is 13 years old, and I don't know when it was changed last, so it can't hurt, I guess.

[DueyT]

Lately I've been getting around 14mpg (down from a previous average of 19) and have noticed a decreased power output. Would it be a safe first assumption that the oxygen sensor might need to be replaced? The check engine light came on briefly once a couple weeks ago, and I havn't done a stomp test yet to see if there's a code stored in the system.

Thanks!

AllanS
91' 535iM
DANG! Just got a 1212 today (#2 O2 sensor) :(

I'm coming up to a Inspection II, I'm thinking both O2 sensors is the way to go?

Cheers,
Duey

p.s. I never get a 1000 at the end of the codes...just repeats 1212. And I can't reset the light with the 10-second pedal bit. ?Que pasa?

[AllanS]

I got a code 1222 - it keeps flashing the same one, with 2.5 second intervals, like yours. 1222 indicates either a lean or rich condition- rich, in my case, because mileage is decreasing along with power. Now, how do I troubleshoot this? Perhaps it could be a airflow restriction? Either way, I already ordered an o2 sensor, so that should help narrow it down.

[AllanS]

Well, I figured it out, sort of.

I had put a small screen in front of the intake in order to keep mice out of my airbox. I took the screen off this morning, reset the mileage computer, and took it to work. In and around town, it was now getting 16mpg, up from 14. I just got off the highway 5 minutes ago, where I reclocked it getting on, and getting back into town, at 22mpg. Power seemed improved as well. So apparently, putting a little screen in front of the airbox can affect airflow and performance... but shouldn't the airflow sensor detect that there is less air entering the manifold, and adjust fuel mixture accordingly, instead of allowing a rich condition (and throwing me codes for it as well)?

Eh, anyway, it's back to normal, and I've got a spare o2 sensor now since I ordered one from BMA yesterday :) I'll be monitoring it as well to see if it fluctuates.