1. An internal combustion engine, comprising a number of cylinders, the or each cylinder containing a four-stroke reciprocating piston, an exhaust conduit, one or more engine operating condition sensors including an exhaust gas sensor in the exhaust conduit for measuring the composition of the exhaust gas, a fuel injection system, and an engine management system for controlling the operation of the engine including the fuel injection system and the air/fuel ratio for at least one cylinder, wherein the engine management system contains engine operation data, the engine operation data being related to expected engine operation with engine fuelling on the correct stroke and/or engine fuelling on an incorrect stroke, and the engine management system is arranged to: a) receive from said sensor(s) respective signal(s); b) oscillate the air/fuel ratio between a relatively rich level and a relatively lean level, the exhaust gas composition varying depending on the air/fuel ratio; c) reverse the direction of change of the air/fuel ratio when the exhaust gas composition is sensed as being indicative of rich engine operation or lean engine operation; d) determine the temporal characteristics of the oscillation in the air/fuel ratio; and e) determine whether or not the engine is being fuelled on the correct stroke by comparing said temporal characteristics with said relevant engine operation data.