my check engine light is always on

Thats not true, go to an auto zone or advance auto and have the car read for free to determine whats going on.

Could be that the gas cap has a bad seal or is not installed correctly. Other and as of late, automakers are installing quick fill points which do not have gas caps thus relaying on a small seal at or on the fill point lid. often the spring within the lid which holds the lid shut, can loosen or brake and not seal as it use to.

The gas cap thing would be a po442, slow EVAP leak, in which would turn on the check engine light, but like joemom says, just go and get the codes read. This will tell you were the problem is.

My check engine light is on in my 2006 range rover hse sport