Check engine light came on shortly after a tune up in October 2011. They placed the car on diagnostic machine and the code indicated gas cap needed replacement and this was done ($35). A week later the check engine light came back on. Diagnostic machine gave same code and they told me it must be the charcoal canister. I asked the service writer if he was sure that replacing this part would take care of it and he said, yes. This was replaced (cost $500). About 3 to 4 weeks later the check engine light came back on. Now they are telling me we will have to place the car back on the diagnotic machine and see what code comes up and it could be anything including the cat. Converter. Am I getting a straight story from these people?