I have a 2001 S500. No modifications or major repairs. Pretty well maintained with correct fluids and scheduled maintenance.
CEL was on dash with the P0410 code (secondary air pump issue) and codes for all four o2 sensors (P0130, P0136, P0150, P0156).

I've read posts from every forum even nearly related to this, and I've done these repairs:
Tested Secondary Air Pump itself, the relay, and fuse. Pump works, shuts off after a minute when engine warms up.
Replaced all o2 sensors.
Tested Catalytic Converters with temperature gauge upstream vs downstream.
Checked vacuum from engine, and from lines. Replaced most vacuum lines.
Tested check valves, electrical and mechanical. All were good.
Replaced EGR valve.
Cleaned airbox and replaced air filters.
Disconnected battery after repairs and drove until CEL came back on.

Comes back on around 30 - 50 miles after reset. Car drives fine.

Any thoughts?

 

Possibly air passages in cylinder head exhaust ports clogged. I used .105" square string trimmer cord and a variable speed drill to ream them out...
Remove the check valves for access.