Well, after getting no definitive advice from anyone except the Meineke guy, I decided to replace the left cat. Wanting the problem to go away forever, I popped for the genuine Mercedes article 129 490 68 19. Cost was $860 vs $710 for a 50-state "replacement."

Last night I dropped the old cat out and was able to get a better look at the problem.

- there is an unfilled hole on the top of the rear weld that I missed with the Thermo Steel (TS)

- the TS had come off in one place, probably causing a leak

- the exhaust blew several small holes in the TS in various places

See pictures for details. So I am clinging to my theory that even small air leaks or maybe the replacement cat caused the ECU to believe that the upstream sensor was not working.

And a definite lesson is that Thermo Steel is not all it's cracked up to be.

Hopefully, UPS shows up today and this issue gets closed out.

Attached Thumbnails

   

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe