A loose or worn gas cap could be cause the Check Engine Light to illuminate.Google+
Car Problem Reports
Cadillac STS Check Engine Light Due to Loose or Worn Gas Cap
Cadillac STS Problem
Average Mileage: 54,000 mi (28,000 mi - 80,000 mi)
Visitor, 2007 Cadillac STS, 28,000 mi
STS V8, "check engine light" comes on occasionally, dealer states no trouble codes are evident, has no idea how to repair, GM will give no assistance, after creating a complaint file on it.
Visitor, 2006 Cadillac STS, 80,000 mi
Last seven months the check enine light keeps coming on. dealer shows no interest in repair or trouble shooting.