During a cold start, there is more fuel in the air-fuel mixture. This also means that there is more unburned fuel in the exhaust gases. The secondary air pump uses this to heat up the catalytic converter. The secondary air pump pumps clean air from the air filter, directly into the exhaust. Here, the air reacts with the unburned fuel, which creates heat. As a result, the catalytic converter becomes warm enough within a few minutes to properly filter the exhaust gases. As soon as the catalytic converter is up to temperature, the valve to the air pump closes, and the air returns to the combustion chambers as normal.
Secondary air pump defective
When the air pump is switched on, this is audible. It sounds a bit like a vacuum cleaner in the engine compartment. A broken secondary air pump has no direct effect on the operation of the engine. However, it is important that it works properly. The catalytic converter filters harmful substances and particulate matter from the exhaust gases. If the air pump is not working properly, there will be more harmful substances than permitted in the exhaust gases during a cold start. If the pump is defective, the lambda sensor measures this and sends a signal to the ECU. The engine warning light on the dashboard will then light up. With an OBD scanner you can read out what the exact problem is. You can order a new secondary air pump cheaply at Winparts. With your licence plate number, you can find a suitable pump for your car.