If an APU emergency shutdown switch has no effect after 11 seconds, what should be checked next?

When the APU emergency shutdown switch has no effect after 11 seconds, the next critical step is to ensure that the cockpit electrical power control panel assembly battery switch is on. This is essential because if the power control system is not activated, the command to shut down the APU cannot be executed. The operations of various systems, including the APU's emergency shutdown functionalities, rely heavily on the availability of electrical power.

Without power, signals from the emergency shutdown switch may not reach the APU, rendering the switch ineffective. Ensuring the battery switch is in the "on" position is fundamental for diagnosing and troubleshooting the issue effectively. This step helps confirm that the systems are operational and can respond to commands, which is vital before proceeding to other checks or actions like verifying fuel levels or inspecting for damage.

Addressing the other choices may be necessary, but ensuring the electrical power supply is fundamental, as it impacts the entire shutdown process.