Tighten loose fuel cap
Average repair cost: $0.01
A gas cap that is loose, missing or damaged can trigger the check-engine light and also allow gas to evaporate. This problem accounted for 5.4% of repairs, compared with 7.1% in 2013. CarMD said the low cost of this repair means that most mechanics are likely diagnosing this problem, but expect to spend $30 if the gas cap needs replacing.