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Code P0456

My 2001 Jaguar XJ8 has about 75,000 miles on it. For the last few months, the check engine light comes on. Each time I have had it checked, the Code was P0456. Apparently, the most common problem with this code is a defective gas cap. The current gas cap is a result of a recall when the car was about 2 years old. The dealer ran a test on the fuel line recently and determined that there was no leak. Is this often a false-positive code, or should I spend the money to have it checked out further? I am not very mechanical myself and know little to nothing about these things.