what happoens when the charcoal cannister needs to be changed and is not? on 2008 Toyota Corolla

A diagnostic was run on this vehicle on January 23, 2013 because engine light was on. Said gas cap needed to be changed. Engine light came back on. Now they say the charcoal cannister needs to be changed. Why didnt it show up the first time?

Asked by for the 2008 Toyota Corolla
The charcoal cannister was probably the problem in the first place. Vapor recovery systems errors are diff. to diagnose and find the cause without a "smoke test". The cap may or may not have been a problem. At least the true issue has probably shown up now.