Hi there guys, I've got a 1997 Legacy Outback Wagon, 2.5, AT. I also have a 1996 Subaru Legacy, 2.2, 5spd. In both these cars, I have the common P0420 code that says "Catalyst Bank 1 Below Threshold"

 

I get that part, obviuosly my cat is old and now working properly, but which cat is Bank 1? The one on the Y-Pipe up front? Or the one that is more rearward from the Y-Pipe?

 

Any help would be greatly appreciated. You guys alwyas seem to have the answers here!

Catalyst bank one is the first converter. The problem could be due to the fact that this converter is not working properly or because there is damage to it or to the exhaust system between exhaust manifold and rear converter. It could also be caused by a bad downstream O2 sensor : the one on the rear converter. It is this sensor that tells the ECU if something's wrong upstream. It could be also be caused by a bad connection to this downstream sensor. I dont think the ECU can tell the difference between a good sensor sending info that things are no up to spec and a bad sensor sending faulty info. Maybe others can dwell deeper into this.

Good luck!

So the first converter is the one that is a part of the exhust manifold?

 

I am just assuming that the converter is bad, it seems that many have this problem and try to get away with replacing sensors, but just end up replacing the cat anyway.

That would do it too. Or a lazy ox sensor.

So the first converter is the one that is a part of the exhust manifold?

Sort of. On my car the firs cat (upstream) is connected just where the Y pipes meet.