They're called O2 sensors for the purpose of monitoring oxygen present in the spent gasses.

The zirconia sensors, once they reach operating temperature, do just that.

I with a richer than normal mixture, not enough oxygen on the O2 sensor will drop the voltage telling the ECU to lean it out, and vice versa.




Closed Loop

The computer uses the oxygen sensor's input to regulate the fuel mixture, which is referred to as the fuel "feedback control loop." The computer takes its cues from the O2 sensor and responds by changing the fuel mixture. This produces a corresponding change in the O2 sensor reading. This is referred to as "closed loop" operation because the computer is using the O2 sensor's input to regulate the fuel mixture. The result is a constant flip-flop back and forth from rich to lean while keeping the average overall fuel mixture in proper balance to minimize emissions.

When no signal is received from the O2 sensor, as is the case when a cold engine is first started (or the 02 sensor fails), the computer orders a fixed (unchanging) rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to regulate the fuel mixture. If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor's signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions. A bad engine temp sensor can also prevent the system from going into closed loop because the computer also considers engine temperature when deciding whether or not to go into closed loop.

Closed loop mode is during cruise engine demands, for better fuel efficiency and emissions, and at higher throttle settings the ECU goes into open loop to a pre determined [richer] mixture setting for better power.