97 outback ej25, 3000 miles into a 4000 mile roadtrip we had a check engine light, pulled over and checked it with a scanner, "PO420 catalyst efficiency below threshold (bank 1)" cleared the code and the light stayed off the rest of the trip.

Not really worried about it at all, the car just passed emissions, it's good for another 2 years in that respect, and it runs good.

Just wondering what the cause might be.

Search the code, there is lots of good information hear and at the outback form.  CarDoc has a very extensive article on it.

 

You did the right thing, start with the easy stuff first:

- Fuel Filter

- NGK plugs and plug wires if needed

- Check for air and vac leaks