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: 101,243 mi (60,000 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
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, 2004 Cadillac DeVille, 75,000 mi
replaced cap multiple times from dealer. finally they must have felt sorry for me and told me to clean the cap really well...toothbrush & water...dry well with lint free cloth. It worked!!! have to do about every six months as error codes starts again. Still doing @ 102,000 miles...looks like they would have fixed this by now!!!!!
Visitor, 2001 Cadillac DeVille, 88,353 mi
Check Gas Cap displayed on 'info'. Tried tightening cap several times, light came back as soon as I started the engine. Saw on another forum that the cap may need replacing...will attempt that as the first option.
Visitor, 2001 Cadillac DeVille, 76,000 mi
Happens by accident (my wife or kids) all the time...always make sure cap is tight when you are done filling the tank.