Have a 2001 Subaru Legacy Wagon with ~165K on it. History - Intermittently over the course of a few years, and always in the cold winter, I've had the CEL come on always with a P0420 code - Catalyst Efficiency Below Threshold (Bank 1). Reset and the code would disappear for at least a year. When it came on more frequent I spoke with the dealer and they said replace the Cat(s). Present - Mieneke did the Cat in early December for $200 less than the dealer wanted for just the part. Went a month and now have P0420 codes. Cleared and they return in a matter of days. Have had these read at both Auto Zone and Advanced Auto - both have recommended replacing the O2 sensor as that is the problem and not the Cat in their opinion. Cat is under warranty so I can always go back and have that done again. Anybody have similar problems and which O2 sensor might be the problem - I'm concluding the front?

 

the 420 ONLY has to do with the rear O2 sensor after the cat.

you can replace the sensor since the cat is new OR do a search for rear o2 sensor fix. You'll end up using 2 sparkplug anti-foulers to move the sensor away from the exhaust stream. ~$15 vs. ~$150 for a new sensor.

We get that code when we remove cats, its a universal code for ALL OBDII compliant cars. The fix is also universal.

 

It'll probably be the rear O2 sensor as that is used to measure cat efficiency. But if you haven't changed your front O2 sensor till now then that might be a good thing to change as well. You can hook the front one up to an oscilloscope and have a look at its response time, I just replaced mine and it's crossing the 0.45v line about 5 times more frequently. From memory the new O2 sensors have an average life of 130k km, or 80k miles.

As for OBDII and cat efficiency:
"The OBD II system compares O2 sensor readings upstream and downstream of the converter. If the downstream readings start to match those of the upstream O2 sensor, it indicates a drop off in operating efficiency and sets a catalyst code P0420."
http://www.aa1car.com/library/converter.htm

 

you could have a crack in the exhaust pipe or they reused the gaskets when they replaced that cat. Exhaust leak could be the cause.

 

Just to update the original thread. Ended up replacing both the front and back O2 sensors and the CEL has stayed off. Mistakenly ordered the back sensor when I thought I was ordering the front so I kept the one and properly ordered the other. On 2 occasions I read an additional code indicating knock sensor problems - bought that also but never installed. Appears the O2 issues were causing the knock sensor code. All is well with 174K and still going strong. Be a few years before I buy the next one.