check engine light on

My wife has a 2003 VW Bug and the check engine light stays on all of the time. The car runs great. You can unhook the battery for about 30 minutes and when you hook it back the check engine light will go off and stay off for two to three days. I have not had it checked for the code yet.
Does anyone know what it could be?

Thanks,

theres a lot of things it could be. theres really no way of knowing until you get the codes read.