Check engine light

I got a gas cap code (vacuum fuel system leak very small) at least once a year and was unable to clear it unless I slightly greased the sealing ring on the cap. This year the grease didn’t work so I bought a new cap for $17. Once I got a code indicating coolant temperature was too low. Early warning that the thermostat needed to be replaced. I paid $60 for my code reader and code clearing device, well worth the money. Look for one that gives an explanation rather than just a code number.