The question of whether an O2 sensor can be bad enough to cause drivability
or fuel economy problems without triggering OBDII codes has another data
point.

In the alt.autos.subaru forum somebody asked for help sorting out very
troublesome hesitation after slowing in a 2006 Outback. A respondent had the
same problem in his car, and pressured the dealer to do *something*! The
dealer replaced the O2 sensor (front one, I presume) in spite of lack of
diagnostic codes and the symptom disappeared.

Mike