It of those EPA mandated devices. It comes on for about two minutes when you start the engine. It blows air into the exhaust system, via a SAS valve mounted up high on the engine, to get the engine up to normal exhaust specs sooner. On volvo's the system has a design flaw that allows exhaust condensation to flow by gravity down the hose and collect in the pump. This eventually fills the pump with water and shorts it out. The solution is to drill a small 1/8 " hole in the bottom providing a drain hole. The system also has a small solonid valve on the inside of the radiator shroud. If you have problems with the system down the road (very likely), it's generally the pump, SAS valve or the solonoid