2003 I35 -- 120k miles.

95% of the time the CEL is on...but then it goes off. Car runs no differently 95% of the time. The other 5% the car starts missing probably due to the coil. My question is why would three codes just vanish? It'll stay off for a few days, maybe 100 miles. I always use premium gas.

Car runs mostly fine, per the onboard computer (i know theyre inaccurate) i was getting 21.5 MPG, never more. City or highway didnt matter. I dont have a heavy foot...with winter gas im getting 19. Might not be relevant.

I do have a steady CEL with three codes:

P0305 cylinder 5 misfire

P0430 Catalyst efficency below normal (bank 2) - I understand this could just be an o2 sensor or bad wiring?

P1456 EVAP control system has very small leak

Strange, normally a misfire will cause the CEL to flash, which when flashing it is telling you there is a serious issue and you should address the issue asap. The P0305 could be a bad spark plug or coil on cylinder 5. Have the spark plugs ever been changed?

As for the P0340 code it likely means you have a bad catalytic converter which is likely caused by the misfire, when the engine misfires, unburned fuel gets dumped into the catalytic converter which destroys the catalytic converter.

Of the 5 or 6 times the engine was shaking and shuddering the CEL blinking happened twice. I always had to pull over and turn the engine on and off a couple times. Once or twice it took 10 tries. I bought a coil to replace #5 but I'm going to replace all the spark plugs while I'm in there regardless. I'm not sure if they were replaced but I bought it from the original owner and have a ton of dealer receipts just nothing with that on there.

Sucks aout the cat if thats what it is.

I'm going to clean the gas cap area nice nice