Troubleshooting a P0420 Code
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?