Automotive AFR (Air Fuel Ratio Sensor) or Wide-Range O2 Sensor

This article is an extension of the Oxygen Sensor article. The AFR is
not a regular O2 sensor. This unit is slowly replacing the normal
Zirconium O2 sensor in all modern vehicles, which is why it merits a
separate article.

THEORY OF OPERATION

The early introduction of the oxygen sensor came about in the late
1970's. Since then Zirconium has been the material of choice for its
construction. The Zirconium O2 sensor, as we all know, produces its own
voltage, which makes it a type of generator. The generated varying
voltage shows up on the scope as the familiar 1 Hz sine wave, when in
close loop. The actual voltage that is generated is the difference
between the O2 content of the exhaust and that of the surrounding
ambient air. The stoichiometric air/fuel ratio or the mixture of
air-to-fuel equal to 14.7:1 is the best mixture ratio for gasoline
engines. At this ratio, the combustion process happens with the most
power being generated and the least amount of emissions being produced.
At a stoichiometric air/fuel ratio (14.7:1), the generated O2 sensor
voltage is about 450 mV. The ECM recognizes a rich condition above the
450 mV level and a lean condition bellow it. Therefore, these sensors do
not care about the air/fuel ratio above or bellow stoichiometry or
14.7-parts-of-air to 1-part-of-fuel. It is for this reason that the
Zirconium O2 sensor is called a "narrow band" O2 sensor.

The Titanium O2 sensor was used throughout the late 1980's and early
1990's on a limited basis. This sensor's semi-conductor construction
makes its operation different than the Zirconium 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 K-Ohms when the mixture is lean. As with the Zirconium
sensor, the Titanium O2 sensor is also considered a narrow-band O2
sensor.

As mentioned before, the main problem with any narrow band O2 sensors is
that the ECM only knows that the mixture is slightly richer or leaner
than 14.7:1. The ECM has absolutely no idea as to the operating A/F
ratio outside the stoichiometric range. In effect it only knows that the
mixture is richer or leaner then stoichiometry. An O2 sensor voltage
that goes lower than 450 mV will cause a widening of injector pulse and
vise-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 Zirconium sensors. These sensors are often called by different
names such as, continuous lambda sensors, AFR (air fuel ratio sensors),
LAF (lean air fuel sensor) and wide range 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 range O2
sensor can detect the exhaust's O2 content way bellow or above the
perfect 14.7:1 air/fuel ratio. Such control is needed on new lean
burning engines with extremely low emission output levels. The tighter
emission regulations are actually driving this newer fuel control
technology and in the process making the systems much more complex and
difficult to diagnose.

The wide range O2 sensor looks similar in appearance to the regular
Zirconium O2 sensor. Its inner construction and operation are totally
different, however . The Wide band O2 sensor is composed of a dual inner
layer called "Reference cell" and "Pump cell". The ECM's AFR sensor
circuitry always tries to keep a perfect air/fuel ratio (14.7:1) 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 flow
created in the pump-cell circuit. The ECM senses the current flow 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 flow 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 flow in the ECM's AFR pump-cell
circuit. The ECM detects the reversed current flow and an injector
pulsation-reduction command is issued bringing the mixture back to lean.
Since the current flow 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
Zirconium O2 sensor can not be used to test the wide band AFR sensor.
These sensors are current devices and do not have a cycling voltage
waveform. The testing procedures, which we will go into further along,
are quite different from the older O2 sensors.

The AFR sensor operation can be thought of as being similar to the hot
wire MAF sensor. But, instead of a 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 actually
happens very fast. The current flow 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 ECM)
converts the current flow into a voltage value and passes it on to the
serial data stream as a scanner PID. 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
actually monitor the actual AFR sensor varying current, but the changes
are very small (in the low mA range) and difficult to monitor. A second
drawback to a manual AFR current test is that the actual signal wire has
to be cut or broken to connect the amp-meter in series with the pump
circuit. Today's average clamp-on amp-meter 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.

Another major difference between the wide range AFR sensor and a
Zirconium O2 sensor is that it operates at above 1200 Deg. F (600 C). On
these units the temperature is very critical and for this reason a
special pulse-width controlled heater circuit is employed to precisely
control the heater temperature. 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 flow to maintain the set
operating voltage. The current flow 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 A/F 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 flow
(0.00 Amps) at 14.7:1 A/F ratio. In this case a light negative hump is
seen on the Amp-meter with the reading returning to 0.00 almost
immediately. The fuel correction happens very quickly.

Toyota among others has always been a strong supporter of wide-range AFR
sensor technology. The OBD II regulation calls for an O2 sensor voltage
range from 0.00 to 1.00 volt. In order to meet the OBD II regulation,
Toyota rearranged the AFR sensor PIDs (from the detection circuitry) by
dividing their original OEM PID value by 5. The newer generic OBD II AFR
sensor PID ranges between 0.48 (rich) and 0.80 (lean).

The following summarizes the wide-range AFR sensor operation. The Toyota
AFR sensor is used here as an example, since the operating voltages
change from one manufacturer to another.

 o  The AFR sensor operates at a much wider air/fuel ratio detection
 range. Hence the name wide range.

 o  The AFR sensor provides the ECM with a signal value throughout a
 broad (wide) range of air/fuel ratios.

 o  The ECM current detection circuit voltage (scanner PID) is totally
 the opposite of a regular Zirconium O2 sensor. The higher the voltage,
 the leaner the mixture and vise-versa.

 o  The detection circuit voltage signal (scanner PID) output is
 proportional to the current flow applied by the ECM to the pump cell
 circuit (to keep the operating voltage) and an indicator of the
 air/fuel ratio.

 o  In AFR sensor fuel control systems, the ECM can more accurately
 measure the actual air/fuel ratio on a wider scale. This allows the ECM
 to adjust to stoichiometry much faster.

 o  With AFR sensor systems, the ECM does not cycle (rich/lean) as in
 the older Zirconium type O2 sensor. The output bias or pump cell
 circuit current detection voltage is fairly stable.

 o  With the mixture at 14.7:1, the AFR sensor pump cell circuit current
 flow is 0.00 mA.

 o  The pump cell circuit current flow changes polarity (by polar).

 o  A rich mixture produces a negative current flow in the pump cell
 circuit.

 o  A lean mixture produces a positive current flow in the pump cell
 circuit.

 o  Because the current can flow in either direction, the AFR's ground
 is NOT chassis ground. The AFR sensor uses a floating or ECM ground,
 which could be held at a specific voltage level above chassis ground
 (according to the manufacturer). Some manufacturers call this circuit
 (Signal -).

 o  The actual pump cell circuit current flow pushes Oxygen atoms into
 or out of the diffusion chamber, depending on the direction of the
 current flow.

 o  The detection circuit always monitors the direction of the current
 flow and how much of it is flowing.

 o  Toyota AFR systems show an AFR PID of 3.30 volts at 14.7:1 A/F
 ratio. Each manufacturer uses a different PID voltage value to signal
 the stoichiometric point. Toyota also divides its OEM PID by 5 in order
 to arrive at an OBD II compliant voltage value.

 o  The leaner the mixture, the higher the detection circuit voltage
 value (scanner PID). The richer the mixture the lower the detection
 circuit voltage value (scanner PID).

 o  The ECM tries to maintain a stable voltage level across the AFR's
 sensing tip or pump cell circuit.

 o  The AFR voltage reading on the scanner is not the actual voltage
 across the AFR sensor pump cell. The AFR detection circuit (inside the
 ECM) generates the scanner PID voltage data from the pump cell current
 flow. The pump cell voltage is kept at a stable value by the ECM.

 o  Wide-range AFR sensors are current devices and do not put out an
 actual voltage for their signal.

 o  The current output signal flowing through the AFR circuit is in the
 mA range and can not be measured with a clamp-on amp-meter.

 o  The same factors that affect the Zirconium O2 sensor also affect the
 AFR sensor (contamination, vacuum leaks, EGR failure, heater failure,
 etc).

 o  The AFR's heater operation is very critical to the sensor operation.
 These sensors operate at a much higher temperature than Zirconium
 sensors.

 o  The AFR heater is pulse-width modulated by the ECM to maintain a
 stable temperature.

 o  The AFR sensor heater is usually ON (pulsing) under normal driving
 conditions.

 o  The AFR heater carries more current because of the higher
 temperatures necessary. For this reason the connections are more
 critical so as to avoid resistance in the circuit.

 o  The AFR heater circuit carries up to 8 Amps compared to the
 Zirconium O2 sensor at 1.5 to 2 Amps

These sensors also have the added advantage of being able to have the
fuel control system adjust to any desired air/fuel ratio other than
14.7:1 (Stoichiometry) or lambda 1. This option is especially important
in new fuel control concepts such as lean-burn engines, where the
engine's fuel control changes at cruising speeds from 14.7:1 to a much
leaner 19.0:1 or even higher. The result is tremendous reduction in
emissions and fuel consumption. It is also worth stating that these
leaner engines require special catalytic converter units capable of
reducing the considerable amounts of NOx generated at such leaner (high
temperature) mixtures.