Martynban

Hey im looking for some advice. I had a secondary air pump issue which caused fault codes for the pump. Sure enough when i tested it the pump was bad and did not cycle when i tested it with my vcds cable. So i replaced the drivers side pump. The pump was an aftermarket pump though. I also had to change the one way valve as exhaust gas was coming up the line with the car running. Now i get codes for insufficiency of flow from that side pump
so my question is what determines the flow from the pump. Is the issue with an aftermarket oump or i guess an o2 sensor that monitors it. How do you check these also?