No. 4: Loose fuel cap
Average repair cost: $15.31
Drivers are getting the message that the check-engine warning can be triggered by a gas cap that's loose, missing or damaged. It also allows gas to evaporate. This problem accounts for 3.84% of repairs, compared to 7.1% in 2014.
Tightening a loose gas cap is free, but expect to spend about $15 if the cap needs to be replaced.