In-use vehicles which are high emitters make a large contribution to the emissions inventory. It is not known, however, whether high-emitting vehicles share common emissions characteristics. We study this by first examining laboratory measurements of second-by-second engine-out and tailpipe emissions from a small number of MY90-97 high-emitting vehicles. We distinguish high-emitter types by the behavior of six ratios in low- and moderate-power driving: the engine-out emissions indices (engine-out pollutant to fuel-rate ratios) and the catalyst pass fractions (tailpipe to engine-out ratios) for CO, HC, and NOx. Four general types of high emitter are observed: 1) fuel-air ratio excessively lean, 2) fuel-air ratio excessively rich, 3) partial combustion such as misfire, and 4) severe deterioration in catalyst performance in vehicles where malfunctions of Types 1, 2 or 3 are not predominant. We also find that these behaviors may be chronic, or may only occur transiently. The second step is to determine the prevalence of the four different types of high emitter in the on-road fleet. For this we analyze IM240 tailpipe emissions from a large sample of cars measured in the Arizona inspection and maintenance program. We find that all four types of failure are observed with roughly comparable probabilities.