[Reaper450128]

Last time I was under my SVX (yesterday), there were two sensors, one on each pipe right before each of the front two catalytic converters. If there isn't an individual code for each one, I'd say you're going to have to experiment (i.e.: buy a replacement and plug it in place of each on in turn, disconnecting the battery for a bit in between each trial) to find out which one it is, but there may be a cheaper and more easier way. Also, I'm not sure what year yours is, but OBDII cars should have a third one after the third cat, however in the case of an OBDII system each one should have an individual code to identify it.

I've got the same problem (mine's a '92), but my first two cats have failed along with one of the O2 sensors, and for some reason since I told the shop not to fix the cats (they want $1000US to fix it, I can get a pair of high-flows for about 200-300) he said he didn't need to fix the O2 sensor either. He didn't say which was bad, so I don't know which one to replace on mine either! Good luck from someone with the same woes.

As for what it does: the sensor checks the amount of oxygen in the exhaust gas. Too much oxygen = incomplete combustion = higher emissions and also means that the system needs to compensate for the air coming in with more fuel. A bad O2 sensor will either detect more or less than the actual amount of oxygen in the exhaust, and compensate the system incorrectly, causing high emissions and/or high combustion temps that damage other parts of the exhaust, in my case thats probably what killed the front two cats. Granted, this is the US version, but I think O2 sensors are universal devices, in function at least.