'97 E420, 120K miles, repeatedly displays OBDII code P0410 for the last several months. Air pump and clutch are functional but engine fails pump activation test: right cylinder bank mV barely drops below 40 at start of test and then climbs to > 200 by test's end; left cylinder bank initially drops to 100 then rapidly climbs to approx. 700 mV. Oxygen sensors are factory original. Vehicle was exposed to fuel with high sulfur content for approx. 2 weeks in May '04.

Engine performance has been WNL although idle has become rough as of late, despite plug renewal. Are cats partially clogged?