Basically a P0420 stands for "catalytic converter efficiency below threshold bank 1". The engine control module, or ECM, on your vehicle tests the efficiency of the "cat" periodically, and will set this code when it fails the test. If I remember correctly, the test has to fail twice in succession before actually turning on the check engine light or CEL / MIL. The ECM is comparing the signals from the oxygen sensors in order to determine the health of the catalytic converter. I have found over many years that if this codes has been stored by the ECM, 98% of the time the catalytic converter actually has degraded. What happens next is crucial, you must determine the root cause of the catalytic converter failure. If it's simply old and worn out then fine, but you better make sure that there aren't any misfires, internal oil or coolant leaks, and things of that nature. Also, these year Subaru engines are fairly notorious for having misfires in cylinder #4 due to coolant and/or oil leaking into that cylinder by way of failed head gaskets. So it would be wise to make sure everything is sealed and in proper operating condition before replacing those expensive cats. Let me know if you have any questions.