What does a check engine light indicate?

The check engine light is an important warning indicator in vehicles that alerts the driver to a potential issue that could affect the engine's performance, emissions, or overall system integrity. When the check engine light is illuminated, it signifies that the on-board diagnostic system has detected a fault within the engine or emissions system. This could relate to a variety of issues, such as problems with the fuel system, ignition system, exhaust system, or even minor issues like a loose gas cap.

The existence of this light serves as an important reminder for the driver to investigate further, typically by using an OBD-II (On-Board Diagnostics) scanner, which can provide specific error codes indicating the exact nature of the issue. Ignoring the check engine light could lead to more significant problems, decreased performance, and potentially expensive repairs if not addressed promptly.

In contrast, the other options describe scenarios that do not accurately reflect the function of the check engine light. While a vehicle running smoothly would not typically trigger the check engine light, an increase in fuel efficiency or a routine oil change would not cause this warning light to come on. The check engine light specifically indicates that there is a fault that needs attention rather than the absence of problems or improvements in vehicle performance.