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Water reaching the secondary air injection pump and destroying it. The pump is an electric motor that only runs for a minute after a cold start, and it sits low in the engine bay drawing air from a wheel arch — so water arrives from spray, from washing, or backwards through a failed check valve.

Exhaust condensation is the route people miss. The exhaust side of this system is full of water vapour, and a check valve that no longer seals lets it condense inside the pump housing. So a water-damaged pump argues for inspecting the check valve in the same visit — a new pump behind a failed valve has a known expiry date, which is why these fail in pairs.