I have a 528i 1999 with 170 K miles. I am getting the alarms "secondary air injection system insufficient flow bank 1 and 2".
I removed the smog air pump and run a direct 12 volt to the pump. It ran great with good airflow. I tested the secondary intake valve and it hold the vacuum and with the hose disconnected there is no back flow of exhaust gasses. I replaced the relay and checked the wires at the relay box. I also checked the power at the relay box and its showing 12 volt at the right wires. I tried at a cold engine while starting to measure if i got power at the connector at the smog air pump. I dont get any voltage at cold start and the smog pump doesnt come on. The question is: Does the smog pump has to come on at all cold starts or does it have to be a certain temperature outside for the DME to activate the secondary injection system. Is there an thermostat or anything that could cause the DME not to activate the secondary air injection system? Does the secondary air injection system come on at every cold start, no matter what the outside temperature is at this that particular time?
After more research and finding a great write up, i finally found the fix. The 50 amp fuse #107 was blown. It is located just forward of the passenger front seat under the carpet near the door. I measured the wrong wires at the DME box which made me believe I had power at the relay. Always great to share fixes.