So tonight I got under the car, swapped downstream O2 sensors from side to side, and put some more ThermoSteel on the welds including the front weld (for the first time). Hooked up the PC and took a test drive.

This time B2S2 behaved much differently. It looked a lot like B1S2, staying around .5V instead of dropping down to .1V like it did before. Back in the driveway, I let it idle for 10 minutes or so. As the engine temp came up, B2S2 started to fluctuate slowly around .5V, while B1S2 stayed at about .6V.

I think the car ran a bit more smoothly.

LTFT B1 was .8, and B2 was 0.

Given that both sensors are new OEM Bosch, I don't think the swap caused the change. I now believe that even the tiniest exhaust leak between the sensors can cause these problems. So I am leaning toward replacing the cat with the OEM version which comes as a single piece that bolts to the exhaust manifold and the muffler. I am working up my courage to ask the price of the Mercedes version.

__________________
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