An advanced procedure for off-line power transformer diagnosis has been presented in this paper. Several different diagnostic measurements can be made using only one device. Examples and case studies were discussed to show the variety of defects and degradation mechanisms that can be detected using the procedure. In particular, it was found that the most common OLTC defects can be detected. The measurements are very sensitive to degradation due to long-term aging and to OLTC maintenance errors. A large population of OLTCs was tested, and a substantial number showed contact degradation. Finally, several areas that require careful attention were discussed, i.e., test current amplitude, circuit resistance, secondary short circuiting, and winding configuration.