Abstract An EHC (electrically heated catalyst) of a four stroke motorcycle engine with heat storing material was used to investigate the effect of input energy on the carbon monoxide (CO) conversion efficiency after cold start. The factors studied included the length of the heat storing material, heating temperature and pre-heating time. The stainless steel heat storing material was 15 mm wide and 0.3 mm deep and was installed at the inlet and mid-section of the catalyst. The settings of the parameters were heat storing material lengths of 30 and 60 cm, pre-heating time of 5, 15 and 25 s, a CO setting level of 1.3% and 1.8% and a heating temperature of 140, 180 and 220 °C. It was revealed that for the shorter heat storing material, better conversion efficiency was attained when heating at the inlet. Heating at the mid-section of the catalyst with the same length of heat storing material resulted in less energy stored, quicker heat dissipation and more frequent re-heating required. In contrast, with a longer heat storing material, the temperature rise was more gradual and more heat was absorbed, resulting in a more stable overall temperature development. It was further showed that under the same input energy, the shorter heat storing material provided a higher CO conversion efficiency than the longer material. Further, a threshold total input energy for achieving a CO conversion efficiency above 65% was found to be 100 kJ.