I'm working on my '04 Corolla (114k miles) which
is throwing code P0420 (catalyst system efficiency
below threshold bank 1). I suspect that it is just the
O2 sensors that may be bad rather than the whole
converter. I have not noticed a strong odor from
the exhaust, nor loss of power, and it's getting
around 35mpg hwy. Should I try to just do the O2
sensors? Is there a more reliable way to identify if
the issue is the cat or sensors?
An O2 sensor is a lot easier and cheaper to check or replace than buying a new catalytic converter. You may also want to look for any exhaust leaks, leaking injectors, or timing, as those can also trigger a PO420 code. You can Google "how to test O2 sensor" and "how to test catalytic converter", but just so you know getting an O2 sensor out of the exhaust can be a challenge for a home based DIYer. There's a video on Youtube as well on checking catalytic converters for blockage using a shop vac. Most likely your problem is going to be with the sensor or one of the other issues listed above and not with the cat.