Which of the following is a sign that maintenance might be required on the APU?

High exhaust temperature readings indicate that the APU may be experiencing issues that require maintenance. In a properly functioning Auxiliary Power Unit (APU), exhaust temperatures typically remain within specified limits during operation. Elevated exhaust temperatures can signify problems such as a malfunctioning fuel control unit, insufficient airflow, or issues within the combustion chamber, which can lead to inefficiencies and potential damage if not addressed. Monitoring exhaust temperatures is critical as it reflects the internal operation of the APU and its overall health.

On the other hand, consistent low pressure readings would alert maintenance personnel, but they are not as definitive an indicator of APU issues as high exhaust temperatures. Normal operating temperatures and efficient fuel consumption suggest the APU is functioning well, so these factors would not trigger maintenance needs. High exhaust temperatures are a clear and direct sign that intervention may be necessary to prevent further complications.