Question

On my 1997 Subaru Legacy, which o2 sensor is it? catalyst efficiency below threshold. Front cat has been replaced within the last year

Answer 1

P0420 Catalyst system below efficiency

Before any catalytic is installed the engine performance should be thoroughly inspected, especially the fuel trim, what monitors the catalytic is a 02 sensor, the rear 02 sensor, this is also known as a downstream or catalytic monitor. What can easily degrade the catalytic is motor oil, the proper type is important, which means no synthetic motor oil is to be used, just regular.