Is it true that the 'check engine' light can come on when the gas cap isn't on tight enough?
This is true, but there are countless other reasons for an illuminated Check Engine Light as well.
The EVAP system is designed to keep fuel vapors from entering the atmosphere. The powertrain control module (PCM) monitors this system and sets a trouble code if it detects a leak, such as a loose gas cap. I suggest you have a trained professional, such as one from YourMechanic, diagnose your vehicle to determine the cause of your Check Engine Light.
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