How do O2 simulators work?

[Steamroller]

Can someone please explain how an O2 simulator works? Does it have a fixed resistance, a particular signal output, and/or something else?

Do the creampuff vipers have an O2 sensor before and after the catalytic converters? If so, does the ECU only use the front one for fuel/air optimization, and the rear one for simply detecting that the catalytic converters are in place?

Basically, I am interested in how each of the O2 sensors interface with the ECU, and how the the O2 simulators function to not affect the ECU's ability to optimize the fuel/air ratio.

 

[Ulysses]

Basically, I am interested in how each of the O2 sensors interface with the ECU, and how the the O2 simulators function to not affect the ECU's ability to optimize the fuel/air ratio.

The front O2s pulse at some rate above and below 450mV, depending on the O2 content. The rear O2s will pulse at a rate very close to or at zero, if the catalytic is working correctly. The PCM will measure the rate of the fronts and then the rate of the rears. It will then calculate the ratio of the rear over the front pulses and see how close that is to some predetermined value.

Sims will either be a constant zero, or a very slow frequency pulse, which keeps the ratio at or close to zero. That is the basic operation of a sim. However, a sim has to pass several other tests that determine the condition of the O2 sensor itself.