FAA AMT (Aviation Maintenance Technician) Airframe Practice Exam 2025 - Free AMT Airframe Practice Questions and Study Guide

An increase in system pressure or frost formation on the pipes would typically indicate a functioning freon system, rather than a leaking one. A hissing sound could also be caused by other issues, such as a damaged or clogged compressor. Oil seepage, on the other hand, can be a clear sign of a leaking freon system when the system is not in operation. This is because freon is mixed with oil in the compressor, and a leak in the system could cause oil to leak out as well. Therefore, D is the correct answer as it specifies the indication of a leaking freon system while the others do not.