Secondary air injection is an emission control system. It uses an air pump to move fresh air to the exhaust manifold when the engine is in open loop and to the catalytic converter when the engine is in a closed-loop. This achieves a reduction in hydrocarbon, carbon monoxide, carbon dioxide, and hydrocarbon emissions. A check valve is located in the air supply line; thus, it prevents the entry of exhaust gases into the pump.
The check valve, which rules the airflow, might be be vacuum controlled or electrically. When the PCM (Powertrain Control Module) activates this valve, the vacuum moves to the check valve so that it opens and fresh air flows.
During acceleration, the air pump is deactivated, thus avoiding shots in the exhaust. Therefore, the PCM activates the secondary air system to send fresh air to the exhaust system. Because the Oxygen (O2) Sensors translate this fresh air into a lean condition, fuel settings soon begin to move in the direction needed to compensate. The Powertrain Control Module (PCM) waits for this to happen in a short period during the self-test. If the PCM sees that the fuel trim does not increase in a short time, it interprets this as a malfunction in the air injection system and the diagnostic code P0411 OBDII is set.