The last time we got involved with charcoal canisters was in a Tools and Techniques article on slack tube testing back in September 1990. In that earlier article, we referred to the evaporative emission control system as a charter member of the Ignored Auto Parts Club. As long as the system is doing its job, there is rarely a reason to give it a second thought. In this article, we’ll discuss what the evaporative emission control system does and how it works. A better understanding of this system’s operation should make diagnosis easier when the system it stops doing its job.