[GuusV]

The P0420 is a finicky code. There is a range of "acceptable" parameters that Subaru has set. If the O2 sensor sees a value that is outside of that range, even for just a fraction of a second, you get the P0420. Adding the extension doesn't stop the O2 sensor from working, it just doesn't get the immediate read, but more of a mixed read. Kind of like drinking Tea without stirring the sugar first. The last sip is going to be super-sweet...
Looks like your head gasket is seeping.... (from that photo anyway). If you don't mind a drip here or there, and topping up between changes, there's no harm in continuing to drive it like this. Our 2009 had the same issue. I ended up replacing the headgaskets. (and in case you're wondering: it's possible to do that without pulling the engine)

 

[GuusV]

The $5 fix may be a solution too...

('98-'04) $5 fix workaround! (merged thread)

Hi, I have been searching in vain for an explanation of the $5 fix (mechanical) for the PO420 code that I have seen references to. Would someone be kind enough to point me in the right direction to it please? Thanks

www.subaruforester.org

There are quite a few P0420 threads on here.... May take a little reading to figure out what to do next.

C.

I was thinking that route as well. our 2009 kept throwing that code occasionally and it hasn't done it since adding the adapter.
It's worth the gamble for less than $10 !

Our 2009 would occasionally shudder/stumble and would sound like it would almost stall. It got to a point where it got annoying so I decided to do some cleanup.

I removed the throttle-body/butterfly-valve and cleaned it and every component up to the air-filter. Problem went away. I can't say which component it was that made it run better, but it hasn't done that "stumble" since.

Low idle itself isn't an issue, unless it's paired with "rough running", then it makes it feel like it wants to stall.