To assess the technical state of GTE, various tools and methods of diagnostics including instrumental and analytical ones are applied in operation. The analytical approach towards the assessment of technical state of engine is based on diagnostic models and methods. This work suggests some ways of improvement of GTE diagnostic models and methods to be applied in operation. Special attention has been paid to diagnostic matrices (DM) usage in GTE diagnostics, with the matrices being meant for the detection and isolation of faults in gas path of engine. The diagnostic matrices application experience has shown that their application in GTE diagnostics is limited for a number of reasons. Those reasons have been revealed in the paper, and some ways have been suggested allowing one to use diagnostic matrices in GTE diagnostics without restriction. A dedicated software package for automated construction and analysis of the matrices has been developed.