It has long been recognized that the key to achieving stringent emission standards such as ULEV is the control of cold-start hydrocarbons. This paper describes a new approach for achieving excellent cold-start hydrocarbon control. The most important component in the system is a catalyst that is highly active at ambient temperature for the exothermic CO oxidation reaction in an exhaust stream under net lean conditions. This catalyst has positive order kinetics with respect to CO for CO oxidation. Thus, as the concentration of CO in the exhaust is increased, the rate of this reaction is increased, resulting in a faster temperature rise over the catalyst. On a vehicle this means that by enrichment of the air-fuel ratio at cold start (with sufficient secondary air to provide at least the required amount of oxygen for complete CO conversion), the heat generated by this oxidation reaction can be used to rapidly increase the catalyst temperature to hydrocarbon light-off without any additional source of heat.A system based on this ambient temperature light-off catalyst was constructed for testing on a 2.0 L 1995 model-year federally calibrated vehicle. The exhaust system consisted of a conventional TWC for NOx control, secondary air addition via an air pump, advanced HC traps to delay HC emissions and the ambient temperature light-off catalyst. Following an accelerated aging protocol in California Phase II (CP2) fuel, this system achieved emissions of less than 50% of the ULEV standard over a FTP cycle, reducing the HC emissions by a factor of 5 relative to the OEM catalyst configuration.