What happens when gas cap isn't tightened after fueling?

When a gas cap isn't tightened after fueling, it can lead to fuel evaporation, resulting in potential loss of fuel efficiency and increased emissions. The vehicle's onboard diagnostic system may detect the loose cap and trigger a check engine light. Additionally, it can allow dirt and contaminants to enter the fuel system, potentially causing issues over time. Properly securing the gas cap helps maintain optimal vehicle performance and emissions control.