Do you have a code for major or minor loss? The test event happens when all the perameters are met, Once that happens a valve is closed and another opened, the ecu times how long it takes to drop to a specified value. If within specs it then watches how long it takes to bleed down. The first test is major leak and the second in minor. They can be a real pain to find. I have changed a lot of fuel tank vent valves, located at the tank and a lot of purge control valves along with several tanks. I would think a dealer would be required to diagnose is based on the equipment needed.
Bill