[cardoc]

I have been reading over and over the threads on the p0420 code as it seems to keep popping up on my car. Here is my question could a leaking valve cover gasket cause enough of a vacume leak to cause this code?

Its not a VW, Audi, Porsche, BMW, Mercedes, Volvo, Caddillac or Saab. So
no, a valve cover leak will not cause a 420.

Read though here: http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diag.html

The "fix" GG speaks of is of no use and when there is an issue arising that illuminates the MIL with a 420, you need to find what it is. The list is large, but its better to spend time finding the problem rather than spending money guessing or hiding an impending domino effect.

Just because it runs doesn't mean its running proper.