YAWA: O2 SENSORS
10/18/2007
QUESTION: I’m a bit confused on the number of oxygen sensors on different vehicles. How many O2 sensors are on a typical late-model vehicle?
ANSWER: There could be as few as two O2 sensors or as many as six. It all depends on the engine, the engine management system and whether the vehicle has single or dual exhaust with one or two catalytic converters.
On four- and straight-six cylinder engines, there is usually a single “upstream” oxygen sensor in the exhaust manifold where the individual runners or pipes come together. This allows a single O2 sensor to monitor the exhaust oxygen content from all of the engine’s cylinders (the PCM uses this information to control the air/fuel mixture). On some engines (typically inline six-cylinder engines), there may be two O2 sensors in the exhaust manifold depending on how the runners are configured. This allows better monitoring of the front three and back three cylinders.
On V6, V8 and V10 engines there is an oxygen sensor in each exhaust manifold. This allows the computer to monitor the exhaust oxygen from each bank of cylinders separately.
On all vehicles built since 1996 with Onboard Diagnostics II (OBD II), there is also one “downstream” O2 sensor for each of the main catalytic converters in the exhaust system. The downstream O2 sensor(s) are used to monitor the operating efficiency of the converter(s).
QUESTION: Why are there different oxygen sensors? Don’t they all work or function the same way?
ANSWER: No. The most common type of O2 sensor (zirconia) all work the same way, but there are others, namely titania O2 sensors and “wide-band” O2 sensors. Unheated zirconia O2 sensors are the oldest type. They have one or two wires and take up to several minutes to generate a signal after a cold start because they rely solely on the heat from the exhaust to reach normal operating temperature. Consequently, an unheated sensor may cool off at idle and stop producing a signal causing the engine control system to revert back to “open loop” operation (fixed air/fuel ratio setting).
In 1982, heated zirconia O2 sensors appeared that added a special heater circuit inside the sensor to bring it up to operating temperature more quickly (in 30 to 60 seconds). This allows the engine to go into closed loop sooner, which reduces cold-start emissions. It also prevents the sensor from cooling off at idle. The heater requires a separate electrical circuit to supply voltage, so heated sensors usually have three or four wires.
Titania O2 sensors use a different type of ceramic and produce a different kind of signal than zirconia type O2 sensors. Instead of generating a voltage signal that changes with the air/fuel ratio, the sensor’s resistance changes and goes from low (less than 1,000 ohms) when the air/fuel ratio is rich to high (over 20,000 ohms) when the air/fuel ratio is lean. The switching point occurs right at the ideal or stoichiometric air/fuel ratio. The engine computer supplies a base reference voltage (one volt or five volts, depending on the application), and then reads the change in the sensor return voltage as the sensor's resistance changes. Titania O2 sensors are only used on a few applications, including some older Nissans and 1987-1990 Jeep Cherokee, Wrangler and Eagle Summit.
In 1997, some vehicle manufacturers began using a new type of O2 sensor: the heated planar O2 sensor. This type of O2 sensor has a flat, ceramic zirconia element rather than a thimble. The electrodes, conductive layer of ceramic, insulation and heater are all laminated together on a single strip. The new design works the same as the thimble-type zirconia sensors, but the “thick-film” construction makes it smaller, lighter and more resistant to contamination. The new heater element also requires less electrical power and brings the sensor up to operating temperature in only 10 seconds.
Some newer vehicles are also using a wide-band O2 sensor that is similar to the planar design, but produces a higher voltage signal that changes in direct proportion to the air/fuel ratio (instead of switching back and forth like the other types of O2 sensors). This allows the engine computer to use an entirely different operating strategy to control the air/fuel ratio. Instead of switching the air/fuel ratio back and forth from rich to lean to create an average balanced mixture, it can simply add or subtract fuel as needed to maintain a steady ratio of 14.7:1.
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