An emission control device on some engines that pumps air into the exhaust system so the catalytic converter can "reburn" pollutants in the exhaust. The air pump pumps compressed air into the exhaust manifold and in a few cases to the catalytic converter. The oxygen in the pressurized air helps to burn quite a bit of any unburned hydrocarbons (fuel) and thereby converts the poisonous carbon monoxide into carbon dioxide.
Since the introduction of air pumps air injection systems have become more advanced in nature, sometimes using the onboard computer to control system operation. Some engines use pulse-air systems that do not use a pump. Instead, alternating pressures in the exhaust stream are used to pull air into the exhaust system. Some modern vehicles use a high-tech air injection system using an electric air pump controlled by the vehicle's Powertrain Control Module (PCM).