Control purge and vent valves, observe fuel-tank pressure, and investigate why an EVAP monitor may fail.
Trace the fuel-vapor path from tank to charcoal canister to intake; explain purge and vent valve roles; interpret a simplified tank-pressure response; distinguish a leak, a valve fault, and a pressure-sensor fault; choose a confirming test.
Illustrative, non-OEM model. EVAP monitor enable criteria, valve default positions, pressure units, commands, and test methods vary. Never pressurize a fuel tank or introduce ignition sources; follow service information and use approved EVAP test equipment.
Pressure values are illustrative relative units: 0 = atmosphere, negative = vacuum. They are not vehicle specifications.
The ECM commands valves; a sensor reports pressure. A valve command is not proof that the valve physically moved.
In this teaching model, a sealed system retains vacuum after purge closes. A leak or vent that cannot seal allows vacuum to decay faster.