I have been dealing with a DTC P0153 (Bank 2 O2 Sensor 1 response or ageing) on my recently bought 1996 SL500. The conventional wisdom is that the fix is to replace the sensor. So, since the car has 112K, and three of the sensors were original, it got four new sensors. The P0153 recurred about two weeks later. Some internet searching showed exhaust leaks around the cat to be a possible cause. Sure, enough, there was a small leak at the rear of the generic replacement cat where it had been rather poorly welded into place.

Local advice was that the generic cat should be replaced and that it would not work welding the hole. In fact, the local Meineke guy refused to touch it. So as sort of a science experiment, I used some stuff call ThermoSteel to seal the hole.

Then I hooked up my PC-based scanner, which allows you to get a real time display of O2 sensor activity. An image is attached. What it shows, is less "crisp" change in S1 (upstream) sensor voltage on B2 (replacement cat) compared to B1, and a much lower voltage from the S2 (downstream) sensor.

The B1S2 is running a constant .7 volts, B2S2 about .1, but B2S2 ran at about .5 volts until the car was fully warmed up.

So, is this why the replacement cats don't work, or do I have a different problem?

What should the voltage on the downstream S2 sensor be?

TIA

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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