Q: Check engine light is on in 2008 Mazda 3..already replaced gas cap and tightened it. Light went off after filling tank with high t

There are quite a few reasons why the check engine light illuminates, with an EVAP system malfunction (e.g., loose or defective gas cap) just one possibility. If you request a check engine light diagnostic a certified mechanic from YourMechanic would use a code scanner to retrieve the diagnostic trouble code(s) from your car’s PCM (that is, the specific codes have caused the light to set) and then test the appropriate individual vehicle components or sub-systems (often a sensor, circuit, and the like) to pinpoint the reason why the check engine light has illuminated. Once the faulty part or component is identified, it is explained to your satisfaction and the mechanic will let you know of the cost to repair. If you schedule the recommended diagnostic, the mechanic will resolve this promptly for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.