This article focuses on improving techniques for detecting internal defects in the high voltage oil-filled coupling capacitor (HVOFCC). The purpose of the article is to improve the results of technical diagnostics of HVOFCC to control its technical condition based on analyzes of samples of mineral condenser oil (MCO) from this HVOFCC. The following methods were used: gas chromatography (GCh) in determining the concentrations of the components Н2, CН4, С2Н4, С2Н6, С2Н2, СО, СО2, Н2О in the volume of the operational MCO; determining the dependence solubility of air and H2 in MCO on temperature; diagnosing HVOFCC using the Rogers method; descriptions when clarifying the mechanisms of processes occurring in HVOFCC based on the results of diagnosis after its opening. The concentrations of components (Н2, CН4, С2Н4, С2Н6, С2Н2, СО, СО2, Н2О) in MCO samples from HVOFCC type CMP166/√3-0.014 were determined. The dependences of the solubility of air and H2 in MCO on temperature (in the temperature range of 255...373 K), as well as the solubility of gases Н2, CН4, С2Н4, С2Н6, С2Н2, СО, СО2 in this MCO at a temperature of 20 °C were found using the GCh method. Calculated concentrations of gases Н2, CН4, С2Н4, С2Н6, С2Н2, СО, СО2 in the air above the MCO surface in a sealed HVOFCC with internal defects. The Rogers method was used to diagnose HVOFCC based on the results of analysis of MCO samples by the GCh method. It is shown that the emergence and development of the internal defect “Flashover without Power Follow Through” in HVOFCC is facilitated by defects that have arisen during its manufacture and operation, namely, degradation of the MCO; destruction of the membrane boxes of the expander, penetration of MCO into it, penetration of air from the expander into the volume of MCO; the emergence and accumulation of combustible fire hazardous gases Н2, CН4, С2Н4, С2Н6, С2Н2, СО in the air volume above the MCO surface. The results obtained make it possible to increase the reliability of the results of diagnosing the technical condition of HVOFCC with cellulose solid electrical insulation based on the results of GCh analyzes of MCO samples during life tests or before repair. When conducting further research (after opening the HVOFCC during life tests or before its repair), MCO should be sampled to determine its physicochemical, thermophysical and electrophysical properties and the contents of diagnostic components in it (Н2; CН4; С2Н4; С2Н6; С2Н2; СО; СО2; H2S; Н2О; antioxidant additives; furan compounds).