Although major emission reductions from gasoline vehicles have been achieved with the help of catalytic exhaust gas after-treatment, durability of the systems involved is still difficult to guarantee with high confidence levels. On-board diagnosis (OBD) of the main emission control systems is expected to play a central role in any modern emission reduction policy. An overview of theoretical principles regarding catalytic converter operation characteristics, catalyst activity deterioration and OBD system requirements is initially presented. The most important existing approaches to catalytic converter on-board diagnosis are examined by means of presenting principles of operation, assessment methodologies and practical problems associated with each of them. The techniques examined cover dual lambda sensor, hydrocarbon and temperature measurement methods, some of which have already been applied to conform to transitional OBD regulations. It appears, however, that significant work remains to be carried out in order to develop reliable and cost-effective catalytic converter OBD technology to cover legislation requirements, especially for modern low-emitting vehicles.
