Which of the following is a sign that a vehicle may have emissions issues?

An illuminated "Check Engine" light is a significant indicator of potential emissions issues in a vehicle. This warning light is part of the vehicle's onboard diagnostic system, which monitors various engine functions, including emissions control systems. When the system detects an anomaly or failure in components that affect emissions, such as the catalytic converter, oxygen sensors, or exhaust gas recirculation system, the "Check Engine" light activates. This allows the driver to recognize that further investigation and possible repairs are needed to ensure compliance with emissions standards and to maintain vehicle performance.

The other options—such as a flashing turn signal, low fuel level, and defective headlights—do not generally indicate emissions problems. Instead, they pertain to other vehicle functions or maintenance issues unrelated to emissions control. Thus, if you see the "Check Engine" light illuminated, it's essential to address the issue promptly to prevent potential damage to the engine or exhaust system and to remain compliant with state emissions regulations.