Why is the engine light on in the 2005 jaguar xj8?

The most common reason for the engine light to come on, particularly when the car is running well, is a leaky gas cap. This throws a code "evaporation system leak." For me, however, the evaporation system leask was a hole in the charcoal canister that is part of the emissions system/vapor recovery system. This is a thousand dollar repair because it requires dropping the rear suspension. Ouch.