Originally Posted by mikeydrives1
This code is common from what i read among here, Forester.org, Outback.org and RS25.... Simplest fix is a spark plug fouler which pulls the secondary 02 sensor away from the gas stream, thus reading less emmisions.
For whatever reason or another, it seems common (including my own car). The cat isnt always going bad. My car threw this even with a NEW 02 sensor and cat... it popped up twice on a 500 mile roadtrip (crusing at low RPM for long distances) after the fact that it was replaced , so perhaps its just a bug, at least among older subarus.
These will fix your P0420 for about $4. You can get the foulers at AutoZone.