This paper presents the concept of a multi-dimensional marine engine diagnostic tool. The dimensions of the tool are diagnostic signals, which form a vector in affine space. The distance of the resulting vector from reference vectors for considered technical states of the engine is the result of diagnosis. Moreover, diagnostic signals, derived from the composition of the exhaust gas, are also considered. The chosen diagnostic signals are the nitric oxide, carbon oxide, carbon dioxide and oxygen contents in the exhaust gas and the temperatures behind all engine cylinders of the marine engine. Analyses were based on laboratory tests of a 4-stroke marine engine. The operation of the proposed diagnostic tool has been partly verified by a passive experiment under sea operation conditions of a main propulsion marine engine. (C) 2015 Elsevier Ltd. All rights reserved.