If you have a 5LNB non-SWM dish, the guide data comes from 101 if you are watching a channel on 99/101. And from 119 if you are watching a channel from 103 or 119 If you have a 3LNB dish, or any SWM dish or multiswitch, the guide data always comes from 119.
If you start losing guide data, the receiver gives you a warning. If you lose it altogether, the receiver starts looking for the 101 guide data. In some circumstances, the receiver will actually reset...