Howdy. Check engine light came on the other day. Extracted code P0410, secondary air injection failure. Found the elbow at the check valve for the air injection rotted. Cleared codes, replace elbow. Everything appears fine.

My question is, when I start the car, it's hard to tell if the air pump is actually coming on as it's supposed to. Is it audible? I expected to be able to hear it, but I'm not sure I do. These appear to have been common problem items on 850 models where they were buried under the battery and collected water. But on the 960 they're located high and dry next to the exhaust manifold.

Further, in looking through old posts on the subject, there have been owners who reported better performance but lower fuel mileage with a disabled air injection system? Can anyone explain that? My understanding is the system simply feeds fresh air to the catalytic converter during the first few minutes of cold running. How could it impact mileage and performance?
Thanks,
Richard
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86 740T/87 745GLE