Very simply, this code is used to indicate problems with the vapor recovery in the fuel tank. This system allows the system to recover, control, and rebiurn vapors in the tank resulting from the normal volatility of the fuel. The system, when self testing, will pull a vacuum on the tank, and using the indicated fuel level in the tank, expect a given vacuum reading in a given amount of time, indicated by a sensor in the tank. If these parameters are not met, this code will be set. GM had some problems intially with this system, mostly set by customers unaware of the gas cap not being properly reinstalled after gassing up. Other problems can occur as the vehicle ages, including hose and seal leaks, sensor failures, software problems in the computer, etc. Though a dealer's expertise is not essential to the correct repair of a problem, a thorough understanding of the system's operation is. You are entitled to a correct repair if that is what you paid for; I strongly recommend your revisiting this shop. Ray