What to do if a mechanic's work doesn't fix the problem?

[badjoke]

I've had a CEL that I paid to have diagnosed and fixed. The shop determined that the air mass meter, air filter, and purge valve should be replaced to fix the issue. Two days later on a long drive, the CEL came back.

This could be another part gone bad, and all those parts did need to be replaced, but it's also possible that I paid money to fix parts that weren't the problem. How do I handle that situation? Am I entitled to free or discounted labor for the correct fix, since I trusted their diagnosis? Am I just shit out of luck? I like this shop and don't think they're intentionally ripping me off, but I don't really know how to approach them about what to do going forward. What do you guys think?

[gautam]

Can you read an error code from the CEL? If you post it, people on the forum will be able to chime in with the diagnosis and you'll be able to go back to your mechanic with better information.

I'm not sure about the whole refund/discount thing, that's between you and the mechanic. Negotiate a good deal once you have a good idea of what the problem is.

[Mike@PureMS]

Contact the shop and schedule an appointment to bring it back. Sometimes resolving problems requires a step by step process, and one by one they have to rule issues out in order to get to the next step. Sometimes fixing one issue can reveal or expose another issue entirely. For example fixing a bad or broken DV sometimes can expose a boost leak somewhere else, now that you might have boost again.
Give the shop an opportunity to take a another look at it, as don't automatically assume it's the exact same issue.