A Loose or worn gas cap may cause Check Engine Light to illuminate.Google+
Car Problem Reports
Chevrolet Prizm Check Engine Light Due to Gas Cap Issue
Chevrolet Prizm Problem
Average Mileage: 72,750 mi (15,500 mi - 130,000 mi)
Visitor, 2001 Chevrolet Prizm, 15,500 mi
Check engine light: apparently something to do with where you put the gas in.... emission code; have noticed slight decrease in mileage, but still averaging 36-38 mpg city/highway. It is NOT due to the gas cap... the guy told me I would need to replace the whole 'unit' where you put in gas. I haven't.