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The code P0442 is a small evap leak and P0456 is a very small evap leak. If you already replace the fuel cap then it is vey possible you have a leak in the system or the canister valve is faulty. The problem is that you will need some special equipment to test the evap system. You will need a scan tool and a smoke/leak detection machine which pressurizes the system.
You say that both codes, P0442 and P0456, are evap leaks (one small, and the other very small). The dealer originally said that P0442 is multiple misfire. Who is right?
P0442 is not a multiple missfire. I can tell you that 100%.
I understand your confusion but I guarantee you that P0442 is an evaporative emissions leak. Click on the link.