What does a malfunction indicator light (MIL) signify when it is "on"?

A malfunction indicator light (MIL) being "on" signifies that the vehicle's onboard diagnostics have detected an issue that may lead to emissions exceeding permissible levels. This light serves as a warning to the driver that there could be a malfunction in the engine or emissions control system, indicating that repairs may be needed to ensure that the vehicle complies with environmental regulations.

The presence of the MIL means that there is potential for the vehicle to release harmful pollutants into the atmosphere, which is especially critical in regions where air quality is monitored and regulated. Therefore, it is essential for the driver to address the problem promptly to minimize environmental impact and avoid potential violations of emissions standards.

In contrast, the other options do not reflect the function of the MIL accurately. For example, the light being on does not indicate that the vehicle has passed emissions tests or that all systems are functioning properly. Additionally, it is unrelated to the adequacy of fuel levels.