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P0455 means that a large Evaporative Emissions leak is present. Evaporative emissions is simply fuel vapor. The fuel storage system is sealed so that none of these vapors can reach the atmosphere. It is sealed by the gas cap, fuel pump seal, and lines that connect the engine to this system (controlled by two solenoids).

At times, the engine will draw in these fuel vapors and burn them along with the liquid gasoline so that pressure will not build up in the system, and also at times it will close the solenoids and apply vacuum to the system while measuring a pressure sensor. If the system holds vacuum then it determines that there are no leaks, but if it does not hold pressure, then this code sets.

The most common cause of this is leaving the gas cap off after filling the tank. After reinstalling the cap and driving it (for up to several days) the light should turn itself off if this was the problem.

It's also possible the seal on the gas cap is bad, the filler neck is cracked, the fuel pump seal is bad, one of the lines is disconnected (or split/cracked), and/or one of the solenoids is stuck open.

I've also seen many of the 'locking' fuel caps you can purchase at parts stores that don't quite seal up correctly.

Evaporative leak codes can be a real pain to diagnose, even for a seasoned tech, and sometimes requires the use of a smoke machine to find the leak. I would start with the obvious.. check the fuel cap (or go ahead and replace it as they are inexpensive).. check all of the fuel lines between the engine and the tank. If no problem is found here, you'll likely need to take it somewhere to be diagnosed, as specialty equipment is necessary to manually activate these solenoids, and like I said... a smoke machine may be required.

Good luck!