Check engine light on; codes p0302 and 00420

Yes the p0302 code has returned since changing everything out. What do you mean by a base engine issue? The p0420 code came on the same time as the p0302 code, could that p0420 have something to do with the p0302 code? I only ran additives rebought the engine after the code came on.

If you have a misfire on a particular cylinder, it is likely caused by a faulty spark plug or coil, a fuel injector that is not functioning properly, or a base engine issue. Base engine means a problem with the valve train in the cylinder head or the cylinder in the block. Take a compression test of all cylinders and see if number 2 is lower than the rest. Then do a leakdown test if so to determine if it is a head or block issue. This is after you eliminate the obvious things; maybe swap the number 2 plug and coil with the number 4 plug and coil and see if you get a P0304 now instead, meaning the plug and / or coil could be faulty. As far as the P0420 code goes, something has made the catalytic converter stop functioning properly. Often this results from a misfire, as fuel that is not burned by the cylinder dumps into the exhaust and causes the cat to overheat with all that fuel, damaging internal components. It's why misfires must be solved as soon as they occur to prevent this kind of deterioration. Running the additive through the engine depending on the style and content can harm the cat too; hard to say in this case.