Poor gas milage in '97 Outback 2.5l

I guess the obvious answer is check it.

A few things to consider: 1. An OBD II scanner to read the trouble code(s) is pretty inexpensive (there are a few choices at around $120). Autozone will check if free for you, except in California.

1. The code that comes up may not tell you exactly to what is broken, but it will at least provide a clue to help with the diagnosis.
2. If the light coincides with the O2 sensor change, that's a pretty big clue already. For some cars, there are cases where changing brands causes a CEL. Not sure if this ever happens to Subes though. Anyway the new sensor could be bad.

HTH, David