Hi I have a 2004 RX8 with 55100 miles and the check engine light comes on when i dont drive it for a while and goes away after a few days. I was wondering if the 2 codes are related meaning can 1 code make a nother code trigger. What would cause these codes? would it be bad gas? I always use 93 octane.

 

P0302 Rear rotor misfire detected Flash/ON 1 or 2 Misfire ´
If you haven't changed coils then it's due. Could also be plugs or wires I guess

P0420 can be transient... Mine triggered once with nothing wrong... reset, everything good. Cat looks excellent.