Based on an analysis of model emissions data from vehicles tested by EPA, an algorithm has been developed to simulate cold start and warm-up operation using a vehicle emissions simulation model called VEHSIME. During the first 450 seconds of operation following a cold start, the algorithm applies correction factors to the emission rates in 'map' of emission rates over the full speed and load range of a warmed-up engines. The correction factor increases 'engine-out' HC and CO emissions in proportion to the number of seconds left in the warm-up period. The algorithm also applies a catalyst efficiency factor to the map of engine-out emissions as soon as the fuel consumption since start up reaches the cumulative fuel consumption associated with the catalyst reaching 'light-off' temperature on the LA4 cycle.