96 SL Oxygen Sensors

Provide data to the PCM.

Think about it... why would they put a second one behind the cat, if it didn't provide any data to anything?

BTW -- neither the O2 sensors, nor any other sensor, turns on the SES light. The PCM turns on the SES light -- based on the data that it's receiving from the various sensors.

I'm sure you can find loads of information on this if you Google for it.

My understanding is that the downstream O2 sensor (after primary CAT) gives a check against the Upstream (at exhaust port) sensor. The downstream sensors are supposed to last much longer than the upstream one. The downsteam one also measures the effectivenss of the CAT.

IMHO:

Make money for the O2 sensor companies, who paid lobbyests to brige politicians into requiring more restrictive emmission controls. ;)

They seem to balance and measure the efficency of an engine, and catalytic converter to keep bad emmisions low. If the engine has O2 present at the exhaust, it had enough for the ignition. So I've been told.

tom @

You're close... The second O2 sensor serves as a test for catalyst performance. The theory is that if the catalyst is working correctly, it will absorb any oxygen remaining in the exhaust.

In other words, if the second (or "downstream") O2 sensor detects any measurable oxygen, it sets a PCM code and turns on the SES light.

--words