aka Secondary Air Pump Check Valve aka diverter valve
A faulty Kombi/check valve will cause the Secondary Air Pump to fail, by allowing hot exhaust gasses to enter the pump. The gasses contain water vapor, which will condense and eventually corrode the pump internally.
You must also replace the Kombi Valve at same time as this pump.
This is a new, original Pierburg pump, identical to those sold at a VW dealer for more than twice the price. It is NOT one of the cheap, poor quality Chinese imitation pumps found on eBay and other sites.