Toyota Repair: Toyota Charcoal Cannister, hydrocarbon emissions, emission control system

Question
My check engine light came on twice over a 1-1/2 month period.  I took it to the Toyota dealer and they said my gas cap was loose (no other problems).  Then the light came on again.  I took it back and they said my charcoal cannister needeed to be replaced ($500).  What is this and how would I have known that it needed replacement?  Please advise.

Answer
The charcoal canister is part of the emission control system it is supposed to absorb hydrocarbon emissions from the fuel tank and the engine instead of releasing them into the atmosphere, there could be a leak elsewhere such as a connecting hose in which case the fuel tank does not hold pressure and the computer determines that there is a leak in the evaporative recovery system there could also be a vacuum switching valve problem, if the dealer is absolutely sure this is what the problem is and this will fix the check engine light have them go ahead with the repair and if it doesn't work it's their problem but before doing that..............
There are a couple of things that I need to know to be able to help a little more, what year/model is it, how many miles and what engine?