The relative contribution of pollutants emitted during vehicle warmup has been magnified because the outstanding performance of contemporary catalytic converters has nearly eliminated the pollutants emitted from fully warm vehicles. Using an 1814-kg vehicle, the effects of air-fuel ratio and spark advance on exhaust emissions and fuel consumption were assessed for the warmup portion of the Federal Test Procedure. Probable causes of differences between cold- and warm-engine emissions and fuel consumption were identified.During warmup, HC and CO emissions were minimum at air-fuel ratios of 14.5 and 16, respectively. Minimum fuel consumption also occurred within this range. Both HC and CO emissions were insensitive to ignition timing, but retarding the timing decreased NOx emissions significantly. Disregarding driveability, a warmup air-fuel ratio of 16 was judged to offer the best emissions/fuel economy tradeoff.