fiber_tek Posted February 3, 2011 Share Posted February 3, 2011 The front O2 sensor will make this code show up. Mine just did the same thing and a couple people thought it was the cat and most said to go to the dealer and get the front O2 sensor and replace it. I took that advice and it fixed my issues and the P0420 code is gone. You can only use an aftermarket O2 sensor on the rear sensor, you must use an OEM Subaru sensor for the front. A code for the front O2 sensor will show up when it is completely shot, not when it is failing or near failure. So most of the time the P0420 comes from a failing front O2 sensor. A failing front O2 sensor will make the car run bad, and when it is near failure it won't even let the car start. Quote Link to comment Share on other sites More sharing options...
destey Posted February 4, 2011 Author Share Posted February 4, 2011 (edited) The front O2 sensor will make this code show up. Mine just did the same thing and a couple people thought it was the cat and most said to go to the dealer and get the front O2 sensor and replace it. I took that advice and it fixed my issues and the P0420 code is gone. You can only use an aftermarket O2 sensor on the rear sensor, you must use an OEM Subaru sensor for the front. A code for the front O2 sensor will show up when it is completely shot, not when it is failing or near failure. So most of the time the P0420 comes from a failing front O2 sensor. A failing front O2 sensor will make the car run bad, and when it is near failure it won't even let the car start. Its a brand new OEM sensor Still having the problem, just swapped out the IAC, fuel pump, fuel pump relay (that stopped the clicking).... and it still does it. To summarize I've done: plugs plug wires coil o2 sensors MAF sensor IAC valve TPS knock sensor fuel pump fuel pump relay egr feels smooth 3 tanks of fresh gas through with drygas and chevron techron swapped in set of identical injectors from junk yard new fuel filter I had the codes read and it said p0420 cat system efficiency below threshold. It feels like it has no power under 3000rpms. Slight hills on the interstate I have to shift down to 4th and go at like 4000rpms to keep speed up. Then between 1500rpms and 3000 if you have light load it cuts out like the ignition switch was turned off. If you push the pedal down further it comes back on with a slam (all at once). If you let off the pedal then try again it runs (until it cuts out again 2 seconds later). Anyone have any ideas? I've tried just about everything I can think of except for a new ECU. Doesn't seem like a ECU to me though. Forgot to mention I'm getting terrible gas mileage. Not sure exactly, but it has to be 10-20mpg Edited February 4, 2011 by destey added info Quote Link to comment Share on other sites More sharing options...
johnceggleston Posted February 4, 2011 Share Posted February 4, 2011 it sounds like wide open throttle changes things. what does the ECU do differently at WOT? Quote Link to comment Share on other sites More sharing options...
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