[Flatrater]

I have done several hundred of these problems and I have yet to have one stump me.

Now just because you have no leak under vaccum doesn't mean you can't have a leak under pressure.

Another thing that could show that you have a leak is a faulty fuel tank pressure sensor or you can call it a FTP sensor. This sensor is what determines if you have a leak. What the PCM does is close the vent valve and opens the purge valve starts pulling a vaccum, then the PCM closes the purge and looks to see a change in the FTP sensor, no change will store a P0440.

I have seen the fuel sender seal cause a leak and it is hard to pick up using the Evap test station.

The dealer needs to use the flowmeter on the test station to be sure your car has no leaks and that the FTP sensor changes with pressure and vaccum applied.