Welcome! I'm Ron Z! Make sure to use Reply if needed! And PLEASE remember to RATE my answer at the end!
The P0455 is "EVAP System Large Leak Detected". First, make sure the replacement fuel cap you purchased meets Hyundai specs. If this is a "universal" or "locking" fuel cap, or a "discount" fuel cap purchaed from AutoZone, more than likely, it does not meet specs and will not seal the system correctly. If the cap is known good, then the next best course of action is to have the EVAP sysytem "smoked". This is where a machine forces smoke through the system, and the "Large Leak" will reveal itself. Another very common fail item on these vehicles that can set a P0455 is a faulty Purge Sontrol Solenoid Valve (PCSV). This solenoid should be tested for functionality and the vacuum lines in this area should be inspected for any cracks or holes.
ok, that's helpful. If I had autozone clear the check engine light, what do I need to do to return the car to where the NY state inspection computer can read the car. I tried various methods of driving a drive cycle but the mechanic said the cars computer couldn't be read.
Also very helpful. I think I'm down to one last question - is the smoke machine repair leak something that my regular mechanic can do or do you recommend taking it taking it to a dealer. Can you recommend a dealer in Westchester County, NY?