Are you still wondering what an oxygen sensor actually does?  It exploits a unique fact of combustion technology:  fuel burns most efficiently when an air/fuel ratio is exactly 14.7 to one.  When there's a greater ratio of fuel to air, the combustible mixture is called rich.  When there's a greater ration of air to fuel, the combustible mixture is called lean.  The job of the oxygen sensor is to monitor the combustible mixture to make sure it stays in the perfect ratio in order to lower vehicle emissions and give better fuel economy.  How can it do this?

The oxygen sensor relies on another scientific fact, the "Nernst effect."  Nernst's law gives a way to measure the voltage between two materials in close contact, one of which is a known constant.  When the zirconium dioxide ceramic sensor reaches the high temperatures generated by the car's engine, typically 617 to 662 degrees F, it registers a difference between the oxygen content of outside air (the known constant) and the exhaust gas oxygen content.  This difference is emitted as a voltage signal, proportionate to the difference between exhaust gas oxygen content and that of outside air, and is sent to the ECU (Electronic Control Unit) which measures the electrical switching points of the oxygen sensor voltage as the exhaust gas oxygen content changes. The engine computer reads this signal and adjusts the fuel mixture accordingly in order to maintain the perfect ratio.

So, as the engine runs,  the output voltage changes as the fuel mixtures fluctuates between rich and lean.  The type of engine determines the switching speed of the O2 sensor:  (1) carburetor systems switch 1/second at 2,500 rpm, (2) throttle body injection systems switch 2-3/second at 2,500 rpm, and (3) multipoint injection systems switch 5-7/second at 2,500 rpm.

OXYGEN SENSOR PROBLEMS
Clearly, the O2 sensor will slow with age, contamination or damage, decreasing it's reaction time to changes in the air/fuel ratio.  This may cause higher emissions and greater fuel consumption.

The signs of a failed O2 sensor are:

Contamination can occur internally from:

Contamination can occur externally from:

In OBDII systems, an outright failure of an O2 sensor will set a diagnostic trouble code (DTC), but a contaminated or slow sensor might not trigger a code, making diagnosis a challenge.

REPLACEMENT TIPS
Replacement sensors must be the same basic type as the original (heated or unheated) and have the same performance characteristics and heater wattage requirements.  Installing the wrong O2 sensor could affect engine performance and possibly damage the heater control circuit in the engine computer.

When removing a worn-out sensor, check it for signs of contamination.  Some discoloration is normal, but heavy black deposits indicate an over-rich fuel mixture, dark brown deposits indicate high oil consumption, white or reddish deposits indicate harmful fuel additives, and light colored or grainy deposits indicate a coolant leak.

Bosch replacement sensors have a patented double-layer protection system that provides excellent resistance to exhaust contaminants.  The Bosch patented electrode power grid also contains more platinum than competitive sensors.  Stainless steel laser-welded construction, gold-plated heater contacts, an improved wire seal for all 1-wire sensors, and 100% quality control testing assure reliable, trouble-free operation.

Bosch universal replacement sensors use more part numbers than competitors' universal sensor programs to more closely match OEM requirements.  Bosch offers five different 4-wire sensors and two different 3-wire sensors to provide the closest match to OEM sensor performance.

Bosch also uses a patented connection system that can withstand extreme temperatures, engine vibration and moisture for more than 50,000 miles.