service engine soon on 1999 Cadillac Seville STS

why does my cadillac only turn on when service engine soon light is on..?

Asked by for the 1999 Cadillac Seville STS
I'm not sure I understand this. The service engine soon light is the check engine light. You can read more about it here:

It means that there is a problem with your car that could definitely be serious enough to affect starting.