Definition of what it does:
http://www.unofficialbmw.com/images/...lectronic3.pdf
"Secondary Air Injection is required to reduce HC and CO emissions while the engine is warming up. Immediately following a cold engine start (-10 to 40°C) fresh air/oxygen is injected directly into the exhaust stream. The temperature signal is provided to the ECM by the Air Temperature Sensor in the HFM. The ECM provides a ground circuit to activate the Secondary Air Injection Pump Relay. The relay supplies voltage to the Secondary Air Injection Pump. The single speed pump runs for approximately 90 seconds after engine start up. * Below -10º C the pump is activated briefly to “blow out” any accumulated moisture. Secondary Air Injection Monitoring is performed by the ECM via the use of the pre-catalyst oxygen sensors. Once the air pump is active and is air injected into the exhaust system the oxygen sensor signals will indict a lean condition. If the oxygen sensor signals do not change within a predefined time a fault will be set and identify the faulty bank(s). After completing the next cold start and a fault is again present the “Malfunction Indicator Light” will be illuminated."
I'd also be interested in removing this assuming my tune doesn't give a CEL. I'll check on it...