code P420 keeps throwing cel

Bank 1 catalyst system below threshhold. Can anyone tell me if the o2 is bad or if the cat is getting pluged up! By the way the dealer finally got me the right plug wires to fit my engine. They had to order them straight from the plant.

Could be a downstream o2 sensor. Usually, it's a cat. Get a new one and some new 02's and you should be right as rain

cheapest thing to do is change the 02 sensors, reset the ecu, drive on it, if it doesnt show up within 50-100 miles then your good, if it does, then its a cat,

rear o2 sensor.

rear o2 sensor.

 

Hmm, I have most frequently resolved p0420 by replacing the front o2 sensor on older Subarus. Not always, but usually. P0420 can be a bear to pin down, good luck.

P0420 is Bank 1, which i believe is the rear 02

yes It is the rear I just had this replaced on my car, hopefully will pass inspection tomorrow

Bank 1, which i believe is the rear 02

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.

If its your front O2, which handles AFR for the ECU, you'll also more than likely see a lean/rich condition/code along with the 0420 code. You may also notice your mpg drop and/or have stumbling or rough running of the engine.

 

Barring all that, the rear 02 sensor is likely, since its position in the exhaust system also exposes it more to road debris and the elements, leading to a more likely failure.

Thanks for your replies. Put new o2 in today and will see how it goes.

Just wondering if changing the oxygen sensor solved your p420 issue. My car threw the same code recently.

Did the CEL show up again?

Did the CEL show up again?

 

Yes. Does the CEL still come and go after you replaced the 0-2 sensor?

Thanks for your replies. Put new o2 in today and will see how it goes.

 

So how the result after you changed O2 sensor?

My guess is since he hasn't responded, it may have worked. Has anyone else changed it? If so, results?

 

I'll be changing mine this week. Will let you all know how it goes.

I changed the front o2 and replaced the cat and it got rid of my P0420 code. The cat that was on there was aftermarket, so for all I know it was just the front o2 that was bad.

Please let us know your results after driven 100 miles

My guess is since he hasn't responded, it may have worked. Has anyone else changed it? If so, results?

 

I'll be changing mine this week. Will let you all know how it goes.

 

Please update after replaced rear sensor and driven about 100 miles.

Seems this is a common problem with these cars, mine does it more in the winter months, i was told that the Cat is breaking down and not doing its job, which interns grants replacing the cat and sensors as others have mentioned in this post.

it is usually NOT the cats. they are very high quality and are known to last the life of the car.

 

it is not the cats.

it could be :

sensor

exhaust leak

intake leak

engine mis-fire dump unburned fuel in to the exhaust / cats.

 

not the cats.

My mechanic told me that it could also be "bad gas". I switched gas stations, and pumped some premium a few times, and the code went away on its own.

 

As mentioned above, this code comes around more often during the winter months. Most gas stations sell gas with additives (to prevent fuel line freezing, etc.) during the winter. Coincidence? I think not.

I had the P0420 code come on and off every few days for about 3 months. Finally decided to replace the rear O2 sensor and haven't had a CEL yet. I have driven 400+ miles since with no problems so I think it fixed my problem. Hope this helps someone.

Please update after replaced rear sensor and driven about 100 miles.

 

My apologies. Based on what the mechanics had been telling me, I assumed teh code was P0240 and replaced the rear O2. However, the CEL came back on after a few days so checked it myself using a co-worker's scanner. Code thrown is P2097. Ordered pre-cat O2 yesterday from Amazon.

 

Will see what happens.

for future reference, you can always check your codes at O'Reilly's and Advanced Auto Parts for free :-)

Autozone did mine for free, and even gave me neat print outs with the explanation for the code(s).