I have had CEL code P0420 come up on my 96 Legacy 2.2L with 185K miles on it. I replaced both front and back O2 sensors with the proper Bosch replacements (all numbers matched), and the light comes on repeatedly after about 400 miles of driving. FYI - The threaded bung had torn out of the catalyst on the rear one and I had a muffler shop remove the catalyst and weld a new threaded bung onto it ($70 to remove the catalyst, weld, fit both new sensors, and replace on car - fair enough), and they said that there was no visually obvious damage to the catalyst.

 

Since a replacment catalyst is nearly $700, available only from Subaru, I'd like to troubleshoot to ensure that the catalyst is the actual problem. Besides the catalyst itself, the two most likely culprets that could cause P0420 to show up appear to be (1) bad O2 sensors and (2) an exhaust system leak.

 

I'd like some help in assessing the presence of any exhaust leaks in order to rule them out as a possible problem. What I have done so far is to get under the car when cold and warm and wave my hand around various areas of the exhaust system upstream of the catalyst to see if I feel any "puffs" indicating leaks, but felt none. Is there a better or more thorough way to check for leaks? Another suspicion is that there may be a leak that shows up between 55-70 mph, as there is a different sound during these speeds that sounds somewhat like an exhaust leak, but it doesn't make this sound at any other speeds. In addition, the P0420 has always come back on after driving within this speed range for a period of time

 

Could there be a leak that only makes its appearance at certain speeds? What's the best method to check for exhaust leaks?

First thing that comes to mind is having someone in the car rev the engine to the same rpm it revs at when driving at 70mph (3000-3500) while you're under.

Plug the tailpipe while it's idling. If it keeps running it has a leak. If it stalls it don't.

 

At least so say "da Boyz", Click & Clack on NPR.

 

In the olden days we then found the leak by dripping oil thru the carb and looking for the resulting smoke. I don't know what you do with EFI.

that was pretty much what I was thinking but did not want to mention. I used to stuff a potato in the pipe and drip in a bit of oil quick and look before it stalled.

I have no idea what that would do on a modern car, but it worked on a 63 Chevy.

Hmmmmm, cookie are we showing our age or what? And for a "coked up" engine, your either:

 

1. Go out on the highway and drive like hell with one eye in the year-view mirror.

 

2. Drizzle ice water thru the carb while keeping the revs up so the engine won't stall (hand on the throttle bellcrank).

 

Dang, I'll bet these young'uns don't even know what "oakum" is either!

These Subarus just ain't as reliable as a good T Model dang it.

I would just Seafoam the engine, sure generates plenty of smoke for leak detection.