Replaced cat 11 months ago, p0420 & random misfires happened last week. Whats most probable cause?

Question

Cat replaced 11 months ago. Warranty almost up. P0420 just came back. Started last week and engine rough and stalling. Also random misfires p0300 on 1,3 & 4. Plugs replaced recently checked and good.

Answer 1

P0300 Random misfire

Misfire are the most damaging to the catalytic convertor. If a misfire condition is present you can expect a catalytic code until the misfire condition is resolved.