2005 Impreza Outback Sport 2.5L, na, MT5, "Sue Bee"
Thank you @MikeC5 !What brand of injector cleaner did you use? If you look through this thread or search on 'Seafoam', you will see several times where Cardoc has recommended it. It can be added to the gas tank (one can per tank full) and/or drip fed into the intake manifold using one of the vacuum fittings. I had the P0420 coming up persistently (even after new plugs, wires and new fuel injectors, etc.). I was beginning to think it was finally time to replace the cat but tried the Seafoam in the tank and the light has stayed off for around 1,000 miles now. If you have a laptop computer you can also do some in-depth diagnosis using the free Romraider software. You can log all of the engine sensors while driving and then open the saved file in Excel and plot various parameters (many examples in this thread) that can take much of the guesswork out of diagnosing this code.
I have not tried Seafoam in this car but used it in my 1967 Mustang to clean out carbonization with some success.
The fuel injector cleaner I used was "Lucas brand 'Upper Cylinder Lubricant, Fuel Injector Cleaner and Fuel Conditioner' ". I put in about a quart (I know, too much, I must have been impatient). I usually put in a lot less than that. Never had a problem with it even on my highly finicky Mercedes.
The CEL P0420 did not come on until after I ran it about 500 miles and filled it up a couple of times. When I put in the additive it was at a full tank. Then I got the CEL P0420 at a half tank. Thinking there might be bad gas, I filled it back up with 93 octane premium and have reset the code a couple of times while changing/testing other (cheaper) things...but it always comes back after 60-70 miles of driving. The tank is still at 1/2 full. This car gets great mileage.
I'll have to go for a long drive somewhere tomorrow to use up the remaining half tank with the remaining Lucas additive.
I'd like to download this Romraider software and will look up where it is (I'm sure there's info on a thread here somewhere).