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Hi,I have a question about the downstream O2 sensor function. Does it read temperature only or is it active in engine management like the upstream O2.I am replacing the upstream O2 - vehicle has 150K and mileage is not what it should be. I am not going to replace the downstream if it's function is to measure temperature.

Neither sensor has anything to do with temperature. Google it. I get a ba-zillion hits and the first few do a good job of explaining it.Upstream sensors read the O2 in the exhaust and the ECU trims the mix to make it right.In a nutshell...The downstream sensors are after the cat. In theory, the cat basically soaks up excess O2 when the mix is a bit lean, then lets that excess O2 go when the mix is a bit rich, so the downstream mix should be perfectly burned.If the downstream sensor reading is steady(-ish), the cat is working as it should.If the downstream sensor reading is moving about as fast as the upstream sensor, that means the cat is just "in the way" and not doing what it should.

^Let me explain that.Downstream O2 monitors the effectiveness of the CAT.Upstream O2 (or AF depending on the year) has an effect on the fuel trims

In newer vehicles, the downstream O2 sensor is also used as a sanity check for the upstream in case of an upstream O2 sensor hard fault.Assuming everything else is ok, the downstream O2 will trail the mix around stoich, of course not as fast, a slowly varying signal vs. the rich-lean-rich-lean of the upstream, but it gets the mix closer to the ballpark than running open loop alone.