Abstract: A method for determining a fault condition of a fuel system of an engine, the engine comprising at least one cylinder (C1-C6), the or each cylinder comprising a combustion chamber into which fuel is injected by an associated fuel injector of the fuel system and within which, in use, combustion events occur throughout a cycle of the cylinder, and at least one sensor (A1, A2) for providing an output signal which is representative of the combustion events within the or each cylinder, the method comprising: (i) analysing the output signal from the at least one sensor (A1, A2) so as to identify the occurrence of a combustion event in the or each cylinder; (ii) for the or each cylinder, integrating the output signal to provide an integrated output signal; (iii) calculating a first gradient measurement for a first section of the integrated output signal for the or each cylinder, and (iv) comparing the first gradient measurement with at least a second gradient measurement to identify the presence and nature of a faul