The exhaust gas sensor (more commonly known as a ‘lambda sensor’), is a vital sensor in the engine control’s ‘closed feedback loop’. The ECU uses the sensor's output to correct the fuel’s mixture from what it has been programmed with in the fuel map. This will lean the mixture when the sensor reads rich and richen the mixture when the sensor reads lean.
The sensor output’s range from 0.1 Volts (lean) to 1 Volt (rich). A mixture of 14.7 air/fuel will give a reading of around 0.45 Volts. This is because Lambda sensors produce a voltage signal that reacts to the amount of unburned oxygen in the exhaust. This un-burnt oxygen causes a chemical reaction which in turn creates a voltage; when hot (over 250 degrees c.), The voltage varies according to the amount of oxygen content in the exhaust compared to the ambient oxygen in the outside air. The more the difference, the higher the sensor's output voltage.
The lambda sensor's output voltage doesn't remain constant. It alternates back and forth from rich to lean. Every time the voltage reverses itself and goes from high to low or vice versa. A good sensor on a fuel injection system should alternate from rich to lean about once every second. If this is lower then usually it tells you the O2 sensor is getting old (contaminated with carbon etc) and needs to be replaced.
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