I pulled a P0420 code (Catalyst System; Bank 1: Efficiency Below Threshold) from our 2005 Impreza Outback Sport (EJ253 with 5 speed manual) yesterday. It originally popped up back in October (about 1,000 miles ago) so I reset it to see long before it would pop up again. 

 

Kind of a generic code- Is it more likely an O2 sensor (which I know will throw a different code as we already replaced one of them on this car) or is more likely the catalytic converter itself?

 

Most likely will sell this car in the next year so I hope not to sink a lot into it at this point,as it otherwise runs well.

Almost 100% it's the cat itself.

 

 

There are ways around it, but the only correct fix is a new cat.

Imo,usually the cat fails. But putting in 1 or 2 O2 might fix the problem long enough for you to sell it without a check engine light.

Edited December 23, 20169 yr by golucky66

Cat or even a small leak in the exhaust between the 2 O2 sensors.

Fix any exhaust leaks and general engine tune stuff. Plugs and wires, fuel treatment

 

You can ignore it and run the engine indefinitely with that code. Ive routinely seen cars rack up 100,000 miles with that code on. Common. If you're in an emissions state then you're hosed.

 

The great thing is that while replacing the cat often fixes it the cat is not polluting and will pass sniffer emissions tests (except theyvwknt trst it with the light on.

 

I find it ludicrous that there's a huge market for cats that aren't polluting all because a stupid light comes on. So we're replacing more parts more raw materials more manufacturing more shipping and fuel...not emissions friendly.

 

Tight running engine helps.

No exhaust leaks.

Plugs, wires.

 

Debacle of an issue - you can google it. Cardoc has by far some of the best information and approach to this code but it's too technical for most people and shops won't do it. They'll just throw a new cat on it.

^^^^ what he said - often not the cat. Plus, the cat is expensive so, should usually be the last resort. Can easily be exhaust leak, or rear O2 sensor, other things....

We replaced one of the O2 sensors (can't recall if it's front or rear) . I ended up having my mechanic apply heat to the old 02 sensor because the thing was jammed in there so bad. It threw a different, more specific code then though. Also have had codes for a couple of leaky evap hoses that were replaced. Thought that code had come back again, but the last two CEL's were P0420's.

I'd be looking at some of thebyoass rsistor tricks if possible. Not sure that's possible for 2005+ models though. Hopefully a tune up or sea foam or the like will get you through?

 

I've had vehicles tested in MD with the sniffer tailpipe test that had P0420 codes and they always passed. People that install the antifoulers and bypass the light - all pass emissions. People that keep resetting it until it stays off off enough to test - pass emissions.

 

Older Subarus never needed catalytic converters replaced, the OEM units easily last the life of the vehicles.

 

I've talked to engineers that work at plants that manufacture catalytic converters - they're (at least some, maybe not all) designed to last the life of the vehicle from a catalyst standpoint.

 

As I said it's a sad situation that currently far more converters are being replaced than ever before and many aren't necessary at least from a legal emissions standpoint.

05 OB has had the 0420 for the last 60K miles. I changed both O2 sensors and then called it a day. I went through the whole workup with Cardoc and was determined to be a bad cat. Gas mileage is fine. I live in an emissions state. Bought a cheap Autel $20 code reader and I clear the code and allow the systems to reset then scoot over for the emissions test and I'm good for another 2 years.  Eastern Catalytics sells some cheap cats. Probably only last a few years as the cost is a function of how much precious metal is in the cat. So, if you need to sell, try this bandage and leave no forwarding address.

Cheap exhaust stuff is a good temporary solution. They'll rust out in a couple years.

 

A known good OEM unit is an option if you can find one. Post in parts wanted forum here.