I thought it was in this post, but it's not - make sure to smell for gas around your car after you drive, could have a small leak somewhere that's causing this.


For under $100 you can get a code scanner, which should let you view real time data. And then you can diagnose other issues in the future, and friends cars too.

I've got data on my O2 sensor, one of the banks pulses high and low and the post-cat one is pretty steady if I am remembering right. If you plan to get a tool, i'll try to post a pic of my data as reference.
Your O2 could be responding slowly, which would kill the mileage but may not show up as a fault - I don't know enough to figure that part out though.

The O2 sensor may be $150 or so and i'd recommend having them put it in, as they are a pain to get to, and that's well worth the $50 and no skinned nuckles for that.