You can pull a plug and read the number.
The code for P0303 is the middle cylinder in the back, #3. The problem is either the plug, the coil pack, or the fuel injector.
Swap that plug, then that coil pack with another cylinder, to see if the code moves to a different number for that different cylinder. Then it is going to be that plug or coil pack that you just moved is bad. If you move the plug and coil pack, and the code doesn't move, then it is probably that fuel injector has a bad winding inside and not firing. You can check that with a multimeter. You touch the pins to check for continuity. If no continuity, then the wire winding inside is broken, and you need to change that injector.
For the P0421, search Google for "spark plug non fouler", then do that trick to fix that. Not worth replacing the pre-cats on these cars, it is to expensive. Unless the pre-cat is plugged (which it probably is not) then non fouler the thing.
It could be that you are flooding the pre-cat with fuel from the misfire, and it is throwing the P0421 code, because you are fouling the pre-cat driving around like this.
Forget about the Dealer. If the Dealer changes the plugs, changes the coil packs, and changes the pre-cat. You are looking at thousands of dollars more than the car is worth.
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