95 SL320 Oxygen sensor and Oxygen sensor probe

My 95 SL 320 has an Oxygen sensor mounted before the Catalytic converter and another one which is called an Oxygen sensor "Probe" after the Catalytic
converter. Some months back the CEL came on and code P0141, "O2 sensor heater circuit bank 1 sensor 2" was displayed. I cleared the code and everything appeared to be OK for a while. The CEL came back on again yesterday and I am planning on replacing the Oxygen sensor before the Catalytic converter. I have reason to believe that it is the one that has the heating element. Now I am curious about the function of this O2 sensor "Probe". MY 129 service DVD does not appear to address this part. At least I have not been able to find it yet. Any tips will be appreciated, Thanks Peter
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You have OBD-II setup... they measure the before and after cat exhaust to gauge the performance of the cat converter. It seems that your O2 sensor died because of the heating element inside the O2... these sensors are self heated to get fast and accurate performance when cold.
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Unfortunately the problem appears to be intermittent. I am still trying to verify which of the two O2 sensor causes the problem. I was able to do some more investigation within the 129 DVD ETM section. There is reference to an O2 sensor 1 before TWC and an O2 sensor 2 after TWC. I can only assume that TWC refers to the catalytic converter. O2 Heater activation occurs at ECT (engine coolant temp) >80C for O2 sensor 1(before TWC) and <25C for sensor 2 (after TWC). The freeze frame data for the last TC P0141 indicates that coolant temp was 57C when the TC was set. To make matters more confusing the ETM troubleshooting procedure refers to a O2 Heater relay module which controlls the O2 sensor 2 after TWC. Looking at the schematic diagram this relay appears to support O2 sensor 1 (before TWC)instead. Looks like I will be pulling out what is left of my hair on this one.Things just don't make sense. Does the MB service literature have a track record of errors? Mislabels on schematics?
Peter

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