What is the first step to troubleshoot if the APU fails to start?

The first step to troubleshoot if the APU fails to start is to check the fuel supply. The Auxiliary Power Unit (APU) operates primarily on fuel, and without the proper fuel supply, it will not start. Ensuring that there is adequate fuel and that it is reaching the APU is crucial. This involves checking the fuel levels in the tank, verifying that fuel lines are not blocked or leaking, and assessing whether the fuel pump is functioning properly.

Addressing fuel issues first is essential because even if the electrical systems are intact, or the starter mechanism is operable, a lack of fuel would prevent the APU from starting. Identifying and resolving any fuel supply issues early on in the troubleshooting process can save time and ensure the APU is ready for operation.