Originally Posted by Cutlass327
The newer cats also control NOx (oxides of nitrogen) and also unburnt Hydrocarbons (HC). That is the purpose of the 3-way catalyst. That is also why the ECU switches back and forth between rich and lean - to properly keep the cat charged with sufficient O2, and to also allow the ECU to keep an eye on the O2 sensor for proper operation.
The O2s "switching" is merely the way that they work.
When they read lean, they read low voltage(nearly 0v)... when they read rich, they read higher voltage(up to 1v). A "normal" O2 sensor switches back and forth on it's own... it has nothing to do with the PCM "controlling it" because it is an INPUT... not an OUTPUT.
The PCM takes the input and computes the "average" that it gets... normally switching between 0.150v or so and 0.800v... It should usually average around 0.45-0.50v roughly. The reason they don't go completely to 0 or 1v is so that if it does... then it knows that it's internally shorted to power or has an open circuit... helps to pinpoint what may be wrong with it.
Newer cars have factory "lambda sensors" which are not like the normal O2 sensors... I know that the newer Cadillacs w/ the Direct Injected 3.6L VVT DOHC V6 have them.
Their O2 sensors do not switch... they have an internal resistor that changes with A/F ratio and stays steady... much like a coolant temp sensor. They won't switch and will read a constant 0.45v or so.