Major overhauls include extensive testing: DGA, furan analysis, oil dielectric strength, moisture content, insulation resistance, polarization index, winding resistance, TTR, sweep frequency response analysis (SFRA), and sometimes partial discharge tests. Bushings and OLTCs are inspected and tested separately. Thermal imaging may be used under controlled load. Results are compared to factory baselines and historical data to detect mechanical displacement, insulation aging, or emerging faults, informing decisions about refurbishment, uprating, or replacement.