Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
Cadillac DeVille Check Engine Light Due to Gas Cap Issue
Cadillac DeVille Problem
Average Mileage: 100,158 mi (25,500 mi - 194,000 mi)
Visitor, 2004 Cadillac DeVille, 60,000 mi
Code came up with very small vacuum leak in evaporator system or also Check Gas Cap. I originally fixed the problem by dabbing a small amount of grease on the rim of the gas filler neck. But after a year or so I finally figured out the problem. The gas cap is a poor design. I tore apart the gas cap and found out the seal that is really a like a rubber grommet does not contact the rim of the filler neck properly. I took a piece of single strand copper wire and wrapped it around the back side of the seal (side closest to the handle of the cap) forcing the seal closer to the rim side of the filler neck. Problem solved.
Visitor, 2005 Cadillac DeVille, 75,000 mi
Dealer wanted $79.50 for a new cap. Autozone $12.00
Visitor, 2005 Cadillac DeVille, 125,000 mi
I got the warning. I took the gas cap off, cleaned it and applied some light silicon lubricant and the problem hasn't come back in a couple of years now. I did NOT replace the cap... just cleaned it.
Visitor, 2002 Cadillac DeVille, 110,000 mi
I have replaced the gas cap twice without eliminating the problem
Visitor, 2005 Cadillac DeVille, 100,000 mi
Not fixed, needs new gas cap, needs so much stuff, this is small.
Service engine light on and evap code. Both (99,400 and 14,300)times resolved with new cap. Seems like cap should last longer.
Visitor, 2000 Cadillac DeVille, 194,000 mi
check gas cap comes on on the dash replaced cap 3 times still comes on have reset computer everytime
Visitor, 2001 Cadillac DeVille
appears that after it rains the Check Gas Cap sensor goes on..
Thinking it's a Sensor issue
Visitor, 2004 Cadillac DeVille, 145,000 mi
I replaced the gas cap and it is still and issues. I suspect there is a vacuum leak somewhere around the gas tank.
Visitor, 2005 Cadillac DeVille, 104,000 mi
Tightened gas cap and the light eventually went off.