Check engine Light and P0420 Code

Hello everybody, so I had my CEL come on the other day. (This was about the 3rd time since I've owned the car) (8months) and what I have usually done is put in some fuel system cleaner and get a tank of premium gas and it goes away within a tank of gas, which is fairly quick since i drive 300 miles a week to work. Anyway this time I took it to schucks and did a free scan on it and I received this P0420 code. I can't remember the exact definition but something about emissions efficiency. So my question is if the light has gone off again and I seem to be running alright can I ignore it? Or am I going to be ignoring an underlying problem? I've heard it's the CAT or the rear oxygen sensor. Any advice to a first time Subaru owner?

Thanks a bunch (my apologies if this has been adressed before)

-Facepollution

P0420 is a catalyst inefficiency code. There are a few conditions inwhich this code will be thrown. If the catalytic converter is not burning enough of the unburnt fuel from the cylinders the code can be triggered. The ECM looks to see a specific voltage from the rear O2 sensor. If the Voltage is too high or too low,once the car goes into "closed loop" you can get a P0420. More than likely your O2 sensor is Bad. If the O2 sensor is the original its probably time to replace it. Sometimes the car can go through two O2 sensors by 8 or 9 years.

Its always best to make sure you have good spark plugs/wires/ etc. At worst its a bad CAT, but I would lean towards an O2 sensor.

Also a check engine like for this code can go off, if the ECM sees good Voltage 6 key cycles. So its possible that the O2 sensor is just starting to go south, and doesnt always throw the code

Just check out all the other threads about P420 / P0420 and see...

 

And if it's the cat it's often caused by either bad O2 sensors or bad injectors causing the cat to work overtime. Please note that bad injectors may just mean that they are unbalanced.

Had this code for a long time. changed things like plugs, wires, coil pack, fuel filter and front and rear O2 sensors. Code still comes up.

 

I just passed emissions with no problems whatsoever so i'm just going to leave it for now since the leggy drives great with no problems. I have a 01 LGT but it happens with impreza's, outbacks and foresters too.

I have a 02 legacy L replaced cat both sensors fuel filter plugs and mine is still lit any suggestions?