The function of the metallic after-market catalyst developed by Kemira Oy has been tested in Finnish urban traffic and at the temperatures of various seasons. The test cars had different fuel systems: one was a normal carburettor version and the other was of a multipoint fuel injection type. A simulated drive in an average Finnish town has been used as the driving speed cycle. The driving tests were performed in the cold room laboratory of Neste Oy. The US-83 standard test was performed in the engine laboratory of Kemira Oy for comparison.The results indicate that the function of the catalyst under all driving conditions is more effective when the fuel system is improved. The light-off of the catalyst takes place in less than three minutes, irrespective of the temperature. The scavenging effect of the catalyst is improved by its ability to adsorb components from the exhaust gas, which have not lit-off, also during the rich mixture phase. This phenomenon is emphasized in a car equipped with a normal premixing carburettor.The best scavenging level of carbon monoxide and hydrocarbon emissions is more than 50 % and is achieved at +5°C. The scavenging of nitrogen oxides is more than 80 % and less dependent on external temperatures. The absolute amounts of emissions decrease when fuel systems are developed. The limit values of nitrogen oxide emissions for both cars were lower than the limits given in the US-83 standard.The comparison between the urban traffic driving cycle and the US-83 standard test reveals that for the nitrogen oxide emissions, their conversion in the standard test is good under normal driving conditions for both test cars. As to the CO- and HC-emissions, their conversion is better in an urban traffic driving cycle, especially under cold conditions.