00 Buick Regal SES code 0440

My 00 Buick Regal GS (3800 Supercharge with 62k miles) SES light came on. Was told it is a p0440 code. Gas cap is on tight. Was told by several it could be the 02 sensor. Anyone else have this error? What was it? In case it is the o2 sensor, where is it located on this car? How difficult to change out on a scale of 1 to 5 (5 being most difficult). DH tinkers with older vehicles (but not much with the newer ones) so I am hoping we can fix this ourselves and pocket the difference.

Odb-ii DTC errror code P0440 is generated due to a problem with the evaporative emissions system due to a "gross" leak. I have never heard of or seen a 02 sensor cause the error code. Check the vacuum hoses from the engine compartment purge solenoid to the fuel tank and evaporative charcoal cannister/vent solenoid. From the cannister to the gas tank.

Thanks for the suggestions. DH didn't see anything. We finally took it in. Although we had already checked it, mechanic says it was the gas cap not on tight enough. Thank goodness for an honest mechanic!