The early introduction of the oxygen sensor came about in the late 1970s. Since then zirconia has been the material of choice for its construction. The zirconia O2 sensor produces its own voltage, which makes it a type of generator. The varying voltage will display on a scope as a waveform somewhat resembling a sine wave when in closed loop control. The actual voltage that is generated is a measure of the oxygen that is needed to complete the combustion of the CO and HC present at the sensor tip. The stoichiometric air-fuel ratio mixture ratio for gasoline engine is the theoretical air-fuel ratio at which all of the fuel will react with all of the available oxygen resulting in complete combustion. At or near this ratio, the combustion process produces the best balance between power and low emissions. At the stoichiometric air-fuel ratio, the generated O2 sensor voltage is about 450 mV. The Engine Control Module (ECM) recognizes a rich condition above the 450 mV level, and a lean condition below it, but does not detect the extent of the richness or leanness. It is for this reason that the zirconia O2 sensor is called a “narrow-band” O2 sensor.
The titanium O2 sensor was used throughout the late 1980s and early 1990s on a limited basis. This sensor’s semiconductor construction makes its operation different from that of the zirconia O2 sensor. Instead of generating its own voltage, the titanium O2 sensor’s electrical resistance changes according to the exhaust oxygen content. When the air/fuel ratio is rich, the resistance of the sensor is around 950 ohms and more than 21 kilohms when the mixture is lean. As with the zirconia sensor, the titanium O2 sensor is also considered a narrow-band O2 sensor.
As mentioned before, the main problem with any narrow-band O2 sensor is that the ECM only detects that the mixture is slightly richer or leaner than the stoichiometric ratio. The ECM does not measure the operating air-fuel ratio outside the stoichiometric range. In effect it only detects that the mixture is richer or leaner than stoichiometry. An O2 sensor voltage that goes lower than 450 mV will cause a widening of injector pulse and vice versa. The resulting changing or cycling fuel control (closed-loop) O2 signal is what the technician sees on the scope when probing at the O2 sensor signal wire.
The newer “wide-band” O2 sensor solves the narrow sensing problem of the previous zirconia sensors. These sensors are often called by different names such as continuous lambda sensors (lambda representing air-fuel ratio), AFR (air-fuel ratio sensors), LAF (lean air-fuel sensor) and wide-band O2 sensor. Regardless of the name, the principle is the same, which is to put the ECM in a better position to control the air/fuel mixture. In effect, the wide-band O2 sensor can detect the exhaust’s O2 content way below or above the perfect air/fuel ratio. Such control is needed on new lean burning engines with extremely low emission output levels. Tighter emission regulations and demands for better fuel economy are driving this newer fuel control technology.
The wide-band O2 sensor looks similar in appearance to the regular zirconia O2 sensor. Its inner construction and operation are totally different, however. The wide-band O2 sensor is composed of two inner layers called the reference cell and the pump cell. The ECM’s AFR sensor circuitry always tries to keep a perfect air/fuel ratio inside a special monitoring chamber (diffusion chamber or pump-cell circuit) by way of controlling its current. The AFR sensor uses dedicated electronic circuitry to set a pumping current in the sensor’s pump cell. In other words, if the air/fuel mixture is lean, the pump cell circuit voltage momentarily goes low and the ECM immediately regulates the current going through it in order to maintain a set voltage value or stoichiometric ratio inside the diffusion chamber. The pump cell then discharges the excess oxygen through the diffusion gap by means of the current created in the pump-cell circuit. The ECM senses the current and widens injector pulsation accordingly to add fuel.
If on the other hand the air/fuel mixture goes rich, the pump cell circuit voltage rapidly climbs high and the ECM immediately reverses the current polarity to readjust the pump cell circuit voltage to its set stable value. The pump cell then pumps oxygen into the monitoring chamber by way of the reversed current in the ECM’s AFR pump cell circuit. The ECM detects the reversed current and an injector pulsation-reduction command is issued bringing the mixture back to lean. Since the current in the pump cell circuit is also proportional to the oxygen concentration or deficiency in the exhaust, it serves as an index of the air/fuel ratio. The ECM is constantly monitoring the pump cell current circuitry, which it always tries to keep at a set voltage. For this reason, the techniques used to test and diagnose the regular zirconia O2 sensor can not be used to test the wide-band AFR sensor. These sensors are current-driven devices and do not have a cycling voltage waveform. The testing procedures, which will be discussed later, are quite different from the older O2 sensors.
The AFR sensor operation can be thought of as being similar to the hot wire mass airflow sensor (MAF). But, instead of an MAF hot wire, the ECM tries to keep a perfectly stoichiometric air/fuel ratio inside the monitoring chamber by varying the pump cell circuit current. The sensing part, at the tip of the sensor, is always held at a constant voltage (depending on manufacturer). If the mixture goes rich, the ECM will adjust the current flowing through the sensing tip or pump cell circuit until the constant operating voltage level is achieved again. The voltage change happens very fast. The current through the pump circuit also pushes along the oxygen atoms either into, or out of, the diffusion chamber (monitoring chamber) which restores the monitoring chamber’s air/fuel ratio to stoichiometry. Although the ECM varies the current, it tries to maintain the pump circuit at a constant voltage potential.
As the ECM monitors the varying current, a special circuit (also inside the PCM or Power-train Control Module) converts the current into a voltage value and passes it on to the serial data stream as an OBD-II PID (not to be confused with a PID controller). This is why the best way to test an AFR sensor’s signal is by monitoring the voltage conversion circuitry, which the ECM sends out as an AFR-voltage PID. It is possible to monitor the actual AFR sensor varying current, but the changes are very small (in the low milliamp range) and difficult to monitor. A second drawback to a manual AFR current test is that the signal wire has to be cut or broken to connect the ammeter in series with the pump circuit. Today’s average clamp-on ammeter is not accurate enough at such a small scale. For this reason, the easiest (but not the only) way to test an AFR sensor is with the scanner.
By using a scanner to communicate with the ECM, one can view AFR sensor activity. This data is typically displayed as WRAF (Wide Range Air Fuel), A/F, or AFR sensor voltage. However, on some vehicles and scanners it will show up as "lambda" or "equivalence ratio." If the PID displays a voltage reading, it should be equal to the sensor's reference voltage when the air/fuel mixture is ideal. The reference voltage varies from car to car, but is often 3.3 V or 2.6 V. When the fuel mixture becomes richer (on a sudden, quick acceleration), the voltage should decrease. Under lean conditions (such as deceleration) the voltage should increase.
If the scanner PID displays a "lambda" or "equivalence ratio," the reading should be 1.0 under stoichiometric conditions. Numbers above 1.0 indicate a lean condition while numbers below 1.0 indicate rich mixtures. The ECM uses the information from the sensors to adjust the amount of fuel being injected into the engine, so corresponding changes in the short-term fuel trim PID(s) should also be seen. Lean mixture readings from the AFR sensor will prompt the ECM to add fuel, which will manifest itself as a positive (or more positive) short-term fuel trim percentage.
Some technicians will force the engine to run lean by creating a vacuum leak downstream from the mass airflow sensor, and then watch scanner PIDs for a response. The engine can be forced rich by adding a metered amount of propane to the incoming airflow. In either case, if the sensor does not respond, it likely has a problem. However, these tests do not rule out other circuitry problems or ECM issues. Because an AFR sensor can be relatively expensive (up to $400 U.S dollars), a professional diagnosis is recommended.
Another major difference between the wide-band AFR sensor and a zirconia O2 sensor is that it has an operating temperature of about 750 °C (1,380 °F). On these units the temperature is very critical and for this reason a special pulse-width controlled heater circuit is employed to control the heater temperature precisely. The ECM controls the heater circuit.
The wide operating range coupled with the inherent fast acting operation of the AFR sensor puts the system always at stoichiometry, which reduces a great deal of emissions. With this type of fuel control, the air/fuel ratio is always hovering close to 14.7:1. If the mixture goes slightly rich the ECM adjusts the pump circuit’s current to maintain the set operating voltage. The current is detected by the ECM’s detection circuit, with the result of a command for a reduction in injector pulsation being issued. As soon as the air-fuel mixture changes back to stoichiometry, because of the reduction in injector pulsation, the ECM will adjust the current respectively. The end result is no current (0.00 amperes) at 14.7:1 air-fuel ratio. In this case a light negative hump is seen on the ammeter with the reading returning to 0.00 almost immediately. The fuel correction happens very quickly.