Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
Cadillac CTS Check Engine Light Due to Gas Cap Issue
Cadillac CTS Problem
Average Mileage: 92,482 mi (31,205 mi - 180,000 mi)
Visitor, 2007 Cadillac CTS
at 48000 miles the check engine light has come on 3times the the car dealer replaced a sensor to the gas tank and it came back on. The GM dealer said it was the wire to the sensor. A week later it came back on. They spent 4 hours trying to figure it out. They reset it. Now at 54000 miles it is on again. I am thinking somebody at on star is putting the light on. I had simular problems with my 97 catera oillight. And now my warranty is done. The front passanger shock is making noise. The auto trac and auto stablization is out of control.
Visitor, 2006 Cadillac CTS, 38,618 mi
Cap was loose and caused check engine light. When tested error read as some kind of leak. But, all was needed was to tighten the gas cap and have the error message removed.
Visitor, 2003 Cadillac CTS, 119,000 mi
same problem with my car check engine light due to gas cap
Check engine light came on took to dealership and they told me the gas cap was loose
Visitor, 2003 Cadillac CTS, 85,000 mi
I replaced the gas Cap and corrected the problem.
Visitor, 2004 Cadillac CTS, 125,000 mi
This may be a cap issue, but it also may be # 3 clyinder. It will be expensive to find out