I got the dreaded p0420 on my 04 WRX Wagon that I got about 9 months ago. I am pretty confident the dealer that sold it to me cleared it so it wasn't noticeable if you didn't know to look for it when you test drove it. Came on about 200 miles after I bought the car. Didn't realize the nightmare it was or I would have gone striaght back to them. 6 months later still fighting it I realize I really botched it waiting to long to put it on them.
So, first attempt was replace the rear 02 sensor. Cleared the code, came back 200 miles later (basically one cycle I think).
Second Step was new rear Cat, got a Magnaflow made to exact OEM specs according to them and it was the exact same size and whatnot. Cleared the code, came back in one drive cycle.
Step Three, replaced the front 02 or fuel air sensor depending on who you ask. Cleared the code, came back in basically one drive cycle.
Step four, replaced the front Cat, Dealer said this was the problem, so did a cat shop who checked the temperature. Cleared the code. Came back after several drive cycles or about 900 miles of driving. The cat shop says that there is a precat before the turbo? Anyone know anything about this. The car had 164k on it when I got it. It has 170k on it now. There is no indication of any modifications what so ever, one owner car, mostly dealer serviced. Had Subaru cats on it when I got it. Not even the air filter has been modified so I seriously doubt any tune or exhaust mod has ever been done. Replacing the front cat actually did improve the mileage which makes me think this was definitely bad and contributing or would throw it. THe car runs well, though occasionally I get a slightly rough idle. I have put a temp gun on the manifold and all four cylinders are running about the same temp so I don't think there is a cylinder not firing or anything causing that, it could really be fine there and I am just overly sensitive as I am frustrated trying to figure this out. Not sure who to turn too, the Dealer just pulled the code and said put a new one of these on. No actually diagnosis like checking the cat with a temp gun. Thoughts?
Oh, it has a new timing belt and water pump, new ac/belts, etc. I haven't changed the plugs yet, that was gonna be next.