So my NA '06 Legacy threw a code / check engine light a few weeks ago. I drove it for a week and it disappeared on its own. Then it came back a week later. I took it to my mechanic and he used a code-reader to diagnose code P0420, and then reset it. A week later, it's back again.
I was told that it's most likely the catalytic converter, which is $1200 from Subaru, and aftermarket units are not recommended (since they're poor quality, and "give out" quickly. Since my mechanic knows that I'm not going to spend that kind of money on my winter car, he suggested that I tune the car and remove the code permanently that way, and just keep driving it.
My fuel mileage is pretty bad right now, so I want to get this fixed as efficiently as possible to avoid paying the extra fuel costs.
I found this old but useful thread ; http://legacygt.com/forums/showthrea...88#post4307188
Particularly post #8 : (which suggests that it could 1 of 2 oxygen sensors)
Originally Posted by johnegg
bank 1 means it is the driver side bank for v-8, v-6 engines. of for side way v-6 it is in front of the engine not behind it.
most subarus only had 1 bank until the H6 came to be.
sensor 1 it pre cat, sensor 2 is post cat.
so a p0420 bank 1 is going to be the only exhaust on a single exhaust / cats system or the driver side on a dual exhaust / cats system.
a CEL indicating a sensor heater or circuit problem would state something like ''bank 1 sensor 1'', ie: driver side (or only side), front sensor.
the p0420 indicates the cat is not working correctly. there are several possibilities.
1/ the front o2 is old and not doing its job as well as it should. you want the front o2 working as well as possible. it controls the air fuel ratio. as they age they get sloppy and your gas mileage can suffer. this part should be a HIGH QUALITY or subaru part. cheap ones have been known not to correct the problem. subaru brand is a good bet.
2/ the rear o2 is not ''seeing'' the exhaust as well as it should. the a replacement 02 sensor will sometimes correct the issue. this sensor can be more of a generic type. its only job is to make sure you have a cat and it is working. it does not affect how the engine runs.
3/ the cat is bad, but this is not common. these cats usually go the life of the cars.
i would replace the front o2 first and see if that works. this is the most common fix. then i would try the rear o2. i would NEVER replace the cats unless all other solution fail to correct the problem. these are HIGH quality cats and should last a really really long time unless something is done to damage them, either externally or internally.
for some years, 95 - 04 i think, there is a way to trick the computer into think the cat is running efficiently by making a spacer and putting it on the rear sensor. this removes the sensor from the main flow of exhaust and it becomes less sensitive. thus eliminating the CEL. but this should only be used if you have exhausted all possibilities. the engine will still be running wrong and polluting more if there really is a problem.
after 04 they closed the loop hole on the spacer work-around and if you try and use one the ECU will throw a different code.
Should I start by replacing the 0-2 sensor (or both)? How much do they cost? How hard are they to replace on the Subaru? My Honda was a piece of cake, it just screwed into the exhaust and had one plug. Is it that easily accessible on the Legacy too?
Anybody have any experience with code P0420 and/or replacing oxygen sensor(s)?
Any input would be appreciated.