What can contribute to the APU not starting?

In the context of starting issues with the Auxiliary Power Unit (APU), faulty electrical connections are a significant factor that can prevent the APU from starting. The APU relies on various electrical components, including relays, wiring, and control systems, to initiate the start sequence. If there are breaks or corrosion in the connections, or if the wiring harness is damaged, these electrical failures can prevent the APU's starter motor and other essential systems from receiving the necessary power to engage and start the engine.

In addition to electrical connections, while low ambient temperature, low oil pressure, and insufficient airflow could pose challenges or complications during starting, they do not have the same immediate and critical impact on the initiation of the start cycle as faulty electrical connections might. For example, cold temperatures may thicken the oil and affect fluid dynamics, or restricted airflow might decrease efficiency, but these variables often do not outright prevent an APU from starting. In contrast, if the electrical connections are impaired, the entire starting mechanism could fail regardless of other conditions.