|A typical voltage to air fuel ratio curve for a narrow band O2 sensor
The O2 sensor is constantly making a comparison between the Oxygen inside the exhaust manifold and air outside the engine, creating fluctuations in output voltage depending on the oxygen content of the exhaust gasses. The output of the sensor can range between 0 and 1.1 volts, although in normal operation the output voltage range of an O2 sensor is from 0.1 to 0.9 volts depending on the amount of free oxygen in the exhaust gas. When the engine is running rich and has more fuel than needed, all of the oxygen is consumed by combustion and the exhaust gas contains little oxygen. Since the potential difference between the exhaust and air is high under these conditions, the voltage created is higher, greater than 0.45 volts. If the engine is running lean, all fuel is burned, and the extra Oxygen leaves the cylinder and flows into the exhaust. Since the potential difference between the exhaust and air is low due to the oxygen content, the voltage created is lower. In this case, the sensor voltage is 0.45 volts or lower. When the engine is running at a stoichiometric state, the O2 sensor voltage output is constantly in a state of transition above and below the stoichiometric voltage level of .45 volts. The crossing of the 0.45 volt mark is called cross counts. A fully warmed up zirconia O2 sensor will not spend any time at 0.45, merely passing though the .45 volt threshold during its cross cycling as show in the chart of the sensors output voltage above.
The higher the numbers of O2 cross counts, the better the sensor is working. The typical amount of cross counts that should be expected from a healthy O2 sensor is 7-10 times in 10 seconds once the engine is fully warmed up and being revved to around a steady 3000 rpm with no load. The voltage output of the narrowband O2 sensor is very nonlinear when compared to the oxygen concentration and air fuel ratio, so much so that the ECU cannot judge relative richness or leanness of the air fuel ratio. The O2 sensor can only indicate “lean” or “rich” in regards to being on either the rich or lean side of stoichiometric. An engine will only run well under light loads when running at stoichiometric mixtures and the ECU can only adjust the fuel/air mixture to keep the output of the sensor alternating equally between these two values. This state of running where the ECU uses O2 sensor feedback to help control the air fuel ratio is called “closed loop”. When at higher loads and wider throttle opening conditions that requires a richer mixture, the ECU reverts back to its own internal programming to calculate the air fuel ratio. This is referred to as running “open loop”. Since the ECU determines the exhaust gas composition by averaging the high and low swings in the sensor’s output this process adds a delay in the ECU’s ability to adjust the mixture. The term ‘narrow band’, refers to the narrow range of fuel/air ratios to which the sensor responds.
In many cars, the computer sends out a bias voltage of 0.45 through the O2 sensor wire. If the sensor is not warm, or if the circuit is not complete, the computer picks up a steady 0.45 volts. Since the computer sees this as a null value, it knows that the sensor not up to temperature. It remains in open loop operation, and uses all sensors except the O2 to determine fuel delivery. Any time an engine is operated in open loop, it runs somewhat rich and makes more exhaust emissions, worse fuel economy and more air pollution. To speed the time it take to warm the sensor up to full operating temperature and to assure that the sensor is always at the proper operating temperature, even at time of low engine load, many zirconia sensors have an electrical resistance heater built into them. Since the O2 sensor compares the level of oxygen of the exhaust to the ambient air, it is important to prevent the exhaust side of the sensing element from getting fouled with a covering of carbon, oil residue or metallic fouling from leaded race fuel or fuel additives such as octane booster. If the sensors ceramic element becomes clogged, the potential comparison to outside air is not possible and the O2 sensor will be rendered inoperable. It is also just as important for the outside of the sensor not to be occluded by dirt, mud, grease or any other substance which might block the opening to the reference chamber.