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.
Failure of this one-way check valve allows exhaust gases to enter the Secondary Air Pump
The water vapor found in exhaust gases will quickly corrode and destroy the secondary air pump.