Downstream O2 sensor for OBD2

[Eric Parham]

For OBD2 cars with 2 original O2 sensors, what can be done with the downstream sensor (originally after cat) and/or the wires after fitting a racing exhaust? If I do nothing, the check engine light comes on and the car feels much less responsive. That can't be right... can it?

[TBreu007]

Typically, the second O2 sensor is just used for emissions control. If a cat. is malfunctioning, that O2 sensor notifies the driver by a CEL that something is wrong. The second O2 sensor is not used for tuning...that is the job of the first O2 sensor, MAF, MAP, etc.
My last car came with three cats. After removing all three, of course, a CEL was triggered, but there was no loss in performance (actually there was a huge gain). Feeling less power is in your head, or your exhaust is too big, or flows too well (lost backpressure); not because the lack of a proper O2 sensor signal.

That being said, certin new cars ('03 Porsche 911 comes to mind) are programmed to go into limp mode when the second O2 sensor indicates a cat. malfunction. If that was happening in your car, you would have no power at all.

[jhooten]

There is an O2 simulator available for the post cat sensor for many cars that will solve the CEL problem

[Domino]

http://www.mkiv.com/techarticles/oxygen_se...nsor_simulator/

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Mark DiGaetani
#97 ITA CRX
SEDiv

[dyoungre]

The second O2 sensor is also used for long term a/f control (bias - slightly rich, or slightly lean, for catalyst efficiency optimization) as well as determining catalyst function - so it makes sense that it went that the car lost power. The light will light if the catalyst is removed - either put in the simulator, or just figure out a way to get the upstream hego signal to the 2nd hego's input.