Abstract: This paper presents a diagnostic case involving abnormal dissolved gases in the oil of a 220 kV forced oil-circulating air-cooled main transformer. During the initial phase of the fault, characteristic gases indicated high-temperature overheating within the transformer, yet the electrical test data for the main body remained normal, creating a diagnostic contradiction. An innovative sequential correlation analysis method linking "cooler engagement status with oil chemical data" was developed and implemented. This approach precisely pinpointed the fault source to the submerged oil pump of cooler No.5. dismantling inspection confirmed burn damage inside the oil pump. The paper systematically details subsequent control measures and their outcomes, effectively preventing potential misdiagnosis-induced transformer casing removal and factory return. This case provides a systematic methodology and scientific experience for addressing similar faults.