What additional step may be necessary if the APU continues to run despite the APU MASTER switch being OFF?

When the APU continues to run after the APU MASTER switch has been turned OFF, it indicates that there is an alert condition requiring immediate action to ensure safety and operational integrity. In this scenario, the necessary step is to manually force shutdown the APU.

This action is crucial because leaving the APU running without the appropriate switches being engaged can pose risks such as overheating, fuel leakage, or electrical system failure. Manually shutting down the APU allows the crew to take control and safely terminate its operation, preventing potential mechanical failures or hazards on the aircraft.

The other choices, while they may seem relevant, do not address the immediate issue effectively. Checking the electrical circuit might identify the underlying problem but does not ultimately stop the APU from running. Performing the engine start procedure is irrelevant in this context since the intent is to shut down the APU, not initiate another system. Requesting technical assistance may be necessary depending on the situation but does not directly resolve the issue of the APU continuing to operate when it should not.