WHY DOES THE CHECK GAS CAP WARNING KEEPS LIGHTING UP?
AND THE CHECK AIR BAG on 2002 Cadillac DeVille

WHEN I START ENGINE THE CHECK GAS CAP LIGHT GOES ON AND NOW THE CHECK AIR BAG LIGHT ALSO .WHAT CAUSES THIS AND HOW MUCH IS THE REPAIR

3 answers
The check gas cap could be from low pressure reading due to the gas cap not being tightened properly after a fill-up and may go away in time. If not, there is a leak in the vapor recovery system such as a cracked or dislodged vacuum hose or a rusty filler neck. Check air bag light is either a loose wire in the sensors or a failing clock spring on the steering wheel.
have it scan tested for codes and go from there
you have 2 seperate issues evap leak of some kind and a srs issue. seek diag and est