It will be helpful if you post the code. Since they told you to replace the gas cap, it is likely an evaporative emissions code, which is the system that stores gas tank vapors in a charcoal container until they can later be introduced to the intake manifold, where they can be burned via the combustion process rather than have the vapors vent directly into the atmosphere. A common code that occurs is a small leak code, which "could" be caused by a loose or faulty gas cap and is often diagnosed at such. Because it's the easies "fix," it's often incorrectly diagnosed as such. Far more common is a faulty vent control valve, which is bolted to the charcoal canister. That said, there are a lot of things that can cause evaporative emission system codes and the only way to properly identify the cause is to follow the factory service manual procedure for the specific trouble code set. This often requires a scan tool which can control the vent control valve and monitor the evaporative emissions pressure sensor, a Mity-vac tool, an adapter for the evap system test port, a multi-meter and/or a "smoke machine." There are also testers to check the gas cap, which would prevent misdiagnosing one as being faulty.