Bought a 00 civic Si mostly stock...2 days later I get a check engine light with codes p0303 p0304 and p0420...the car starts up and runs fine besides feeling a slight misfire at idle, when driving the car it feels ok until u get on it and almost feels like it loses power....found out there are a lot of things that can be checked to solve this problem but I don't want to just throw parts at the car....has this happened to anyone and do u have any ideas where I should start

 

Do a full ignition tune up. If this solves the misfire issue, then replace the cat.

 

When I buy a used car, I always start with spark plugs. They can tell you a lot about the car.

RonJ's suggestion about full tune up is spot on as you must pull them plugs in the process.

Tune Up, Compression test, fluid changes, Fuel filter (if you don't do that with the tune up), PVC valve and air filter, then timing belt and water pump. I think I got them all for the unknown history car purchase.

That gives you a solid base line to continue maintaining the car.