When should a low air pressure warning signal activate?

The low air pressure warning signal is designed to ensure the safety of both the driver and other road users. It activates when the air pressure in the air brake system drops below a certain predetermined safe level. This is crucial because insufficient air pressure can lead to brake failure, which poses significant risks while driving. When the air pressure falls below this threshold, the warning signal alerts the driver to the need to address the issue before it escalates into a dangerous situation.

In contrast, the other conditions mentioned may not necessarily signal a low air pressure situation. For example, while the vehicle is parked, the air pressure may still be at a safe level, and the engine being off does not directly correlate with low air pressure; air pressure can remain stable even with the engine not running, provided that the system was properly charged before shutdown. Lastly, applying the brakes might temporarily lower the air pressure as the system releases air, but it does not by itself indicate that the pressure has fallen below safe levels.