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An emission system leak is very difficult to diagnose and different on each car.
it could range from an actual leak, a faulty gas cap, rotted out filler neck, vent solenoid
or even a fuel tank pressure sensor.
The system has to be smoke tested using a smoke machine to determine if there is a leak or not then all the electrical solenoids tested.
I would not recommend replacing any parts with out the smoke test.
But if you want to experiment replace the gas cap and have the code(light) reset and see what happens that would be the cheapest part. if the light comes back on the smoke test is a must.