Great DIY for a non-working secondary air pump.
Under the CPO warranty I had my Secondary Air pump replaced, but continued to throw CEL codes due to low secondary air flow.
If the pump is working, and making the leaf blower sound for 2 minutes after a cold start, use the secondary air pump to blow Seafoam into the exhaust ports and valves. This will clear the secondary air passages enough in the engine to allow the second O2 sensor to detect a higher secondary air flow.
The 540i engine is even more prone to the problem, but here's the general idea of what happens:
The carbon burn off using Seafoam on the secondary air system is huge and has kept the CEL off since I first started this cleaning.
Since the Seafoam can only be blown into the ports while the engine is cold this DIY must be performed as you first start the car in the morning.
Step 1: Remove the clamp from the hose between the secondary air pump and the valve to the engine
Step 2: Pour Seafoam into the hose. The hose will hold just over an ounce of Seafoam.
Step 3: Reconnect the hose and run the car 5 seconds so the air pump blows the Seafoam into the exhaust port.
Repeat this fill and restart 3 more times only running the engine 5 seconds so the secondary air pump runs for the 3 fills.
Then let the engine sit 15 minutes and warm up the car. There will be significant amounts of Seafoam carbon burnoff in the form of white smoke. The carbon burnoff will clean exhaust areas in the path to the O2 sensors that measure secondary air flow.