Abstract: A method of analyzing exhaust gas emissions from an internal combustion engine is modified by the provision of a data process or connected to an exhaust gas analyzer and adapted to monitor changes in exhaust gas concentrations and to identify changes indicative of the state of operation of a catalytic system provided in the engine exhaust delivery system. The catalyst-on condition is identified by reference to the characteristic shape of the plot of the gas concentrations against time. Similar identification of catalyst-off conditions is provided by corresponding analysis of concentration changes and concentration values. The system can distinguish between various causes of the catalyst-off condition.