Check engine and cat converter

I own a 1999 subaru outback wagon limited. About 6 months ago the knock sensor needed to be replaced. Which I did. Then car was still running like crap, so I decided to change the spark plug cables and spark plugs. Car ran excellent until last week the check engine light came on. I went to Autozone and had the code pulled. The code was Po420, so took the car to a muffler shop. They said the engine is misfiring at high rpms. Before I did the tune-up cylinder #1 was misfiring, I thought I had corrected the problem. They told me that I needed to find the problem before replacing the converter. I don’t see how it misfires at high way speeds, the car runs great. I forgot to note before and after the tune-up the converter made a bad egg smell when traveling up a steep grade. I want to know if I should spend the 100 bucks for their diagnoisitic service or spend the money on replacing the converter. I had a feeling the converter was going bad back when I replaced the knock sensor

How long did you drive it when it was “running like crap?” That may be when the damage to the cat was done, and you may be okay to just replace the cat now since you fixed the misfire.

If you just do the replacement, you may find yourself doing it again. Before I replace the converter, I would want the rest of the system working right.

ALL engines misfire, a little, all the time. When the misfire(s) get frequent enough (they would cause 150% of the maximum allowable emissions [calculate]), a trouble code (P0300, P0301, etc.) is set in the engine computer and the check engine light is turned on. Lesser misfire(s) can occur and NOT turn on the CEL.
You can test parts, which might cause misfire, by swapping them. Swap the spark plug and wire, and fuel injector #1 with another cylinder. Disconnect the battery (for 5 minuets) to clear the code. Take a short trip. If the CEL comes on, take the car to an auto parts store for a free scan. If the code moved to the other cylinder, that shows the swapped parts were faulty.

My wife and I bought this car used, off a lot. They always make cars look new, except the unknown true condition. I never drove a subaru before, for a year I felt it mis-fire (how it felt). My best description is the firing order skipped, ie 1-3-2-mis etc… At that point it ran okay I guess, no engine light. I knew somthing was wrong but wasn’t sure what it was. I know what misfire is, on previous cars I owned I always check distruber cap, plugs and wires, which I guess should have been done with the subaru minus no distrubuter cap.
Any ways after 2 years of owning car check engine came on. Computer generated a knock sensor code, then after I fixed that computer pop back on about 3 weeks later. This I new for sure it was a misfire on #1 cylinder. I attempted to change spark plug wires, and the boot which goes over the spark plug broke off. At that point I knew I had swap spark plugs and wires.
Sorry for a long drawn out response, however going over the events which lead up to where I am today.

Thanks for the long and clear response. I think that you will be okay to just replace the cat now. You have already resolved the misfire problem. It misfired for one year plus, that was long enough to fatally wound the converter, it just took it a while longer to die.

Okay I never got car fixed, just drove it, about (2 months). The check engine light went out and hasn’t come back for 2 weeks now. In my state of Nevada our county has a fuel additive added to our fuel from NOV. till Feb. It seems to me that may have been my problem. waiting for the weather to warm up a bit so I can check the engine and clear all the codes. I know disconnecting the battery will do but I want everything cleared out.