The most common issue caused by a damaged, cracked or missing gas cap in modern vehicles is that it inhibits the fuel system's vapor evaporation system, which triggers the check engine light to turn on. Having a damaged or missing gas cap can potentially cause decreased fuel efficiency, and in areas of the United States subject to emissions standards testing, a damaged gas cap will cause the vehicle to fail the emissions test.
Damage to a gas cap can occur to the rubber seal, the gasket or the plastic o-ring. Cracking or breaks at any of these places can potentially cause a leak in the fuel vapor evaporation system and engage the check engine light. If left unfixed, over time a leak in the system can cause a decrease in fuel efficiency and potentially hide other leaks in the system.
Emissions testing is designed to measure and limit fumes and emissions issued by a vehicle. A broken gas gap and potential fuel vapor leak will cause a vehicle to fail most emissions standards testing. Fuel vapor occurs when fuel turns from liquid to gas, often with change in temperatures. On modern vehicles, a system is in place to capture the vapor until it can be safely purged to limit vapors released to the atmosphere.