An experimental investigation on the cold-start idle condition of an electrically heated catalytic converter on a four-stroke engine has been carried out. The preheated temperature of the catalytic converter and the level of CO emission of the catalyst were studied, and other measured quantities included the exhaust gas temperatures at the mid-section and at the outlet of the catalytic converter and the concentration of CO. The preheated temperatures of the catalytic converter were varied from raw temperature (non-preheating) to 100, 140 and 180 °C. The levels of CO emissions were set at 1.0, 1.5 and 2.0 per cent. The catalyst was tested with and without heat-storing material around the heater. The experimental results revealed that the catalytic converter with a preheated temperature of 180 °C triggered an early catalytic reaction. A high-level CO setting has the same effect, with the optimum value at approximately 2.0 per cent.