I originally had the P0420 code which, when checked, proved to be a faulty downstream O2 sensor. So that was replaced. The code was cleared from the system before the problem arose with the P0303. I took the ignition coil from the 3rd cylinder and swapped it with the one on the 4th cylinder, ran diagnostics again and it changed the code to P0304 thus telling me it was a bad ignition coil. That was replaced and just for the sake of doing it I also replaced the spark plugs. The system was cleared of all codes again when it came back up again with the P0420. No other codes show in the system currently or were paired with the ones that came up previously.