Before diagnosing the air system, what is the first step that should be taken?

Before diagnosing the air system in a bus, the first step should indeed be to shut off the engine. Turning off the engine is crucial for safety and for ensuring that the air system is in a stable state for diagnosis. An engine running can lead to accidental engagement of the air system, making it difficult to accurately assess various air system components or check for leaks.

With the engine off, technicians can safely listen for leaks and conduct visual inspections without the risk of unintended activation or movement of the vehicle. It also allows the technician to observe any residual air pressure in the system, which is important for understanding the state of the air brakes.

Other actions, such as releasing the brake pedal or checking tire pressure, while important in the overall maintenance of the bus, would not be the first step when focusing specifically on diagnosing issues within the air system.