On my 2000 S500, I've cleared and had reappear after some driving a "P0410" code for Secondary Air Injection Malfunction. What would be the most likely cause of this on this model? Thanks.

There are at least 5 different things that can cause that code on that model!

First thing, in the morning do you hear the AIR injection pump come on? should sound like a vacuum cleaner for a minute or so.

There is a vacuum switch-over valve that has to open 2 diverter valves & then inject air behind the exhaust valves. AT that point the O2 sensors MUST react to oxygen by dropping their output to less 40mv & stay there.

MANY scanners can ACTIVATE that system for that test!

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Take a look at fuse #31 in the right front fuse box. It's a 40 amp Maxi-fuse towards the front.