My '08 Ford Fusion's check engine light went on about a month ago. It was a P0455 code "Evaporative emission control system leak detected - large leak". I was going to fix it myself, but never got around to ordering the part. Anyway, the light had gone off last week and now I'm wondering if it is even an issue any more?
With some variation based on model year and country of use most faults will latch and self clear. By latching the fault, it is stored and in some cases the "check engine light" also referred to as the MIL (malfunction indicator light) will remain lit after repair until certain conditions are met. The conditions for the fault to clear may involve a certain number of miles driven, cold start cycles completed or passed self tests. As @ Nathan Liddle has stated your fault points to the gas cap as the most likely culprit. If you repaired the fault the MIL may stay lit until the fault self clears. Even after the fault clears it will remain stored for a certain amount of time which also varies with model year,etc. This is useful for diagnosing MIL issues that can come and go like vacuum leaks, bad connections and misfires.
How the computer works on most modern cars is like this: It monitors all the sensors in the automobile in near real time. When a fault is detected more than a set criteria per trip, the computer sets of a MIL. So it is possible the computer has not detected that fault in some time (causing the computer to assume systems are normal). So it is very possible the light will come back on if the part is not fixed. It could also have just been a system fault as well. Computers make mistakes too!
So basically, it can come on again if there is an actual issue, or it may not. Eitherway I'd still check it out.
P0455 indicates a large leak as you say. A large leak can indeed be a loose gas cap. The criteria for the code depends on how it is detected. A MAP sensor mis-behaving while driving sets this code on a lot of systems, the rate of pressure over time measured by a pressure sensor is another way to turn the light on. Both the EVAP vent valve and the EVAP purge valve both need to be working correctly. If you get the light/code again you will need a system check of the EVAP, preferable with a bi-directional scanner to test its operation fully. The code must always be regarded as the conclusion the ECU programming, not always a pin point indication of the fault. This means associated systems must be checked as well.