Esteemed and learned usenet readers:
Yes, you read the subject line right. This is regarding an '05 forester. I purchased this car *used* at a very reasonable price from reputable dealer with a 100,000 mile warranty.
The CE light came on within 4 days of my driving it off the lot. The dealer pulled the code and told me it was the O2 sensor. They said they would replace the sensor if the CE light came back on. It did, and they did. The second time, the service guy mentioned something about "slow response" but I was not sophisticated enough to ask more questions.
Everything was fine for about a week, then it came on for a third time. I was going to bring it in, but accidentally ran my battery down by leaving my dome light on all weekend. After the jump, the light was off for a few days, now it's on again.
I just got back from autozone and they pulled 3 codes:
O2 sensor CKT slow response Bank 1 Sensor 2 (2 times) post catalyst fuel trim bank 1 system too lean (1 time)
I always use premium (i.e. 93 octane). I always (as far as I know: now I'm nervous that I'm not) tighten the fuel cap to at least 1 click.
Assuming the 02 sensor really was replaced, and something else is going on, what could this be? I think the Subaru mechanics at the dealer are out of ideas. Spark plugs? Water in the tank? Bad cat? I'm reaching here...