I had a P0455. Took it to my mechanic, They said it was a bad gas cap. Put a new one on. Problem went away for a short time then came back. Put on a new OEM gas cap. Problem went away for a short time. Now it's back again, I clear the code but it returns after a little driving. Sometimes I can go between fill-ups with no problem. I am sure the cap is properly secured after filling up. I can't do a smoke test so what would be the least expensive part to replace at this point that might solve the problem? It's a 2008 G35 Journey Sedan with 99K on it.

 

Throwing parts at a car without actually troubleshooting is literally the most expensive way to try to fix problems. If you want the LEAST EXPENSIVE option then properly troubleshoot the system.

Smoke test, if you're using a Halloween smoke machine or a regular large smoke machine you MUST make a diversion tube because the EVAP system isn't meant to be building higher pressure and even a crappy Halloween machine can make like 10psi. Hook up to the line on the EVAP solenoid, once that end is fully sealed up tap into your main smoke line with a garden hose so you can run the other end out away from the car/shop. Turn the smoker on, test to make sure it's not building too much pressure, you can test with your thumb, it should be moving quite a bit of air but not just blasting out like an air compressor. If it is blasting out air and making the skin on your finger sort of "deform" due to the air pressure then you need another hose added to relieve pressure.

Alternately a gross EVAP leak can usually just be found by hand, go around twisting the hoses at their fittings and see if any are loose, damaged, or simply disconnected. With the car up on jackstands and a mechanics stethoscope you can usually hear a big leak, sometimes you don't even need the stethoscope

 

I got lucky. Replaced the canister purge valve for less than $30 and problem is gone.

 

I realize you solved this but just curious if you had symptoms beforehand, besides just a check-engine light ? A common issue is you can't fill the gas tank up. It will shut off the pump randomly and seemingly creates enough back pressure (???) that the gas pump nozzle won't pump anymore fuel. I had that happen very, very randomly over 2+ years, so rarely that I was able to ignore it. Pffft, just figured it was a screwed up gas pump at the station. Eventually I got a P0455, then it went away. Then it came back. Replaced the gas cap and it went away. Then came back. Replacing the solenoid below the tank (3 hoses, 1 or 2 electrical connections) took care of it.

 

Yes I did have problems with the gas nozzle shutting off well before my tank was full. Did not happen all the time. Since replacing this part my problems are gone. Cheap and easy fix. Took maybe 1/2 half hour.