The purpose of this article is to determine if, in the case of inability to carry out diagnostic tests at engine operating loads in the range of 70÷90%, engine functional systems can generate diagnostic signals in the form of changes in heat balance components. The motivation for working on the selected topic was the need to evaluate the effectiveness of graphical forms of heat balance representation, especially Sankey diagrams, in heat engine diagnostics, using a four-stroke diesel engine as an example. Interpretation of the results was carried out based on simulations performed using Blitz-PRO software and experimental tests carried out on a test bench. For the numerical simulations, the object under analysis was modeled, which was a single-cylinder, four-stroke diesel engine Andoria S320. Both numerical simulations and experimental tests were carried out for three engine operating states: the reference state and two malfunction states: in the air intake system and the fuel supply system. The analyses made it possible to draw conclusions. It was assessed that carrying out the analysis of the Sankey diagram for partial load doesn’t bring significant diagnostic information.