Secondary Air System
Secondary air is used most commonly during the warmup cycle to help burn the rich air-to-fuel mixture needed to run a cold engine. Usually after two to three minutes, the catalytic converter is nearly at operational temperature, so the secondary air system shuts down. On some larger vehicles (e.g. trucks and SUVs), the secondary air system pumps air into exhaust manifolds during warmup and then directs the air into the catalytic converters during normal operation to help "burn off" the high fuel loads required to operate these heavy vehicles.