Our 1997 Outback also does that check-engine On/Off once in a while but runs fine. Our car's muffler recently started making noise. I believe it can be from 2 problems:
1. Old fuel-filter ($70) and fuel pump($20) need to be changed.
2. Catalytic Converter needs to be changed after 80,000 miles. Mine is still working at 154000 miles but it's screaming "change me, change me."
I'm changing those 3.
A new NGK oxygen-sensor, a new set of NGK iridium plugs and a set of borg-warner spark-plug wires are always great to add since they improve the gas mileage by up to 7mpg.