Would it be wise to get a diagnostic check on my P0420 engine code may be a converter Subaru 2005 Impreza?

If the dealer diagnoses the problem and then you have them repair the problem, they usually do not charge for the diagnosis.

You need to go somewhere else. Your upstream sensor is an air fuel sensor and the downstream one is an O2 sensor but it should not be switching at all. .4 volts flat. Suby cats are so expensive but aftermarket cats don't work half the time.

I would have changed an 11 year old A/F sensor before the cat swap. You can't diag a bad A/F sensor that I know of. The heaters usually go out first at ten years.

Perhaps this cat and the original are good. It needs to be diagnosed or you are just guessing. The cat should have a warranty. The person who diagnosed it should have some responsibility. What are the fuel trims doing? What are emissions readings? Did someone smoke test the exhaust system? The problematic exhaust leak in this situation is not noise coming out but actually oxygen being sucked into exhaust. That can happen silently. Without a smoke test you need to visually inspect every square inch of exhaust for even a pinhole leak.

The person diagnosing this does need to know a thing or two or this will be hard to understand. The cat is a chemical reactor and needs a near perfect situation to operate. This is the whole reason we have computer controlled engines and the sole reason these systems were invented to begin with. The o2 sensors are switching because for one reaction to take place you need a little extra O2 and the other reaction requires extra CO. Little things can ruin

You'd just be wasting money. A P0420 code is one of the few codes that actually mean you need to replace the part without further diagnostic work. The P0420 sets because the 02 sensors are working properly and you need a bank 1 converter. If the converter was replaced go back to where you bought it and get another converter. Evidently you bought a POS after market converter which didn't have enough catalyst in it to keep the CEL from coming on. Walker brand after market converters work with a 80% success rate. You have a warranty on that converter if it's only 2 months old.

It was a walker brand I don’t think it was aftermarket. I said the guy didn’t hear a leak. And the code came back in a week

Did you put an aftermarket cat? Check for an exhaust leak. Replace the front and rear O2 sensor. Reset the codes. Code comes back, replace the cat converter