2002 LeSabre loose gas cap warning

The loose gas cap warning message comes on a few minutes after starting, even though the gas cap is on properly. Any ideas as to what the computer is worried about??

The fuel system is not developing the proper vacuum level in the gas tank. The cap maybe faulty or I doubt this one the sending unit seal is faulty. I had the same warning on my 2000 after the sending unit was replaced, the gasket had to be changed to get the warning to go away. It can also be a build up of rust on the end of the filler tube where the gas cap seals against. If it is pitted from rust the gas cap seal will not seat properly causing your problem.