My son is home from college and during his 4 hour trip his 2002 Mitsubshi Galant had the check engine light come on. I took it to AutoZone and they ran diagnostics. The error is "EVAP Emission System Leak Detected. Gross Leak. No Flow Check Vent Valve." As I understand this is part of the EVAP system behind the left rear fender. I took a look and see two selenoid valves. How do I know what to change? How can I diagnose the bad parts? Could this be something as simple as a bad hose?

Thanks in advance.
Gonzo