The AFR will inherit the earth...

The AFR - A wonder of automotive technology. This little sensor will replace all modern O2 or Oxygen sensors within the next few years. Most newer vehicles now employ the AFR or air fuel ratio sensor. It is more precise and covers a wider range of air fuel mixtures. The AFR owns the modern fuel control age.

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 semiconductor 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.

NOTE: Whenever the air/fuel mixture is exactly at stoichiometry (14.7:1) there is no current flow through the AFR sensor. This is precisely what the ECM tries to do with the AFR signal. A properly operating engine will always have very close to 0.00 mA of current flow. The ECM commands more or less injector open time to try and keep the AFR sensor as close as possible to 0.00 mA. A rich mixture will produce a negative current flow and a lean mixture a positive current flow. The actual AFR current flow is extremely small and for this reason, the AFR sensor signal should be monitored with a scan tool.

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.

NOTE: Some diagnostics literatures suggest testing the AFR sensor by goosing the throttle and monitoring the actual voltage. On a good sensor the voltage will snap down-and-up and then go back to its normal level because the ECM will immediately adjust the current to maintain the constant operating voltage. DO NOT use a multi-meter in voltage setting to test the AFR sensor. The only voltage reading that should be used is the ECM’s interpreted voltage value that is displayed as a scanner PID from the pump-current detection circuit.

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).

NOTE: The AFR’s pump-current detection circuit voltage range is the opposite of the regular Zirconium O2 sensor. With the AFR sensor, the lower the voltage value the richer the mixture, and the higher the voltage value the leaner. The OBD II generic AFR PID is called air/fuel ratio sensor and NOT O2 sensor.

The following table gives the values of the Toyota OEM PID, generic OBD II and the actual air/fuel ratio value.

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.

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

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

• 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.

• 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.

• 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.

• 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.

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

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

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

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

• 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 -).

• 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.

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

• 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.

• 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).

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

• 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.

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

• 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.

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

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

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

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

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.

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.

Component Testing

The two more prevalent wide-range AFR sensor system manufacturers are Honda and Toyota. We will use Toyota in this section for explanation purposes. However, the testing procedure is always similar. The only changes will be in the biasing- voltage, which changes from one manufacturer to another. The basic operation is the same. Always learn the system before proceeding to further diagnostics. AFR sensors are also starting to appear in an increasing number of makes and models. It is expected that within the next decade most systems will be of this type. The best way to test the AFR sensor operation is with a scan tool. With that in mind, the use of a graphing-software is highly recommended. This will ensure the quick recognition of the sensor’s operating parameters much faster than simply looking at the numbers. The human brain can process graphical information much better than raw data numbers.


• First, determine that there are no mechanical or air/fuel density problems (vacuum leaks, clogged air or fuel filter, ignition timing, stuck EGR, etc).

• Determine that the AFR sensor bias voltages are within specs. Using a voltmeter, disconnect the AFR sensor and back probe the sensor’s signal wires. Probe between ground and one of the signal wires, then to ground and the other signal wires. The bias voltages should be measured to ground. Measure the bias voltages and compare to specs. The signal + wire is usually the pump cell signal circuit while the signal – is the ECM provided floating ground. (Note: Some manufacturers are using an AFR sensor with 6 wires. Two heater wires, signal +, signal – and an extra two set of wires, which are the pump cell current signal and the calibration current wire.

• Perform a W.O.T. snap test. The scan tool AFR voltage should reach a full low (rich mixture) voltage potential (Toyota – 0.48 volts (OBD II) or 2.40 volts (OEM), then a full high (lean mixture) voltage value (Toyota – 0.80 volts (OBD II) or 4.00 volts (OEM). (refer to fig. – 5).

• If the AFR sensor did not pass the W.O.T. snap dynamic test above, suspect an open or a short circuit to the AFR current circuit. The AFR sensor has to react to the sudden W.O.T. snap test regardless of engine A/F conditions. A non-changing detection circuit is a very strong indication that the AFR circuit is down. Internal as well as external circuit faults are possible. To verify the fault (whether the fault is internal or external) disconnect the AFR sensor and either measure the continuity between the ECM AFR wires and the AFR connector or apply a varying voltage (O2 sensor simulator – 0.00 to 2.00 volts) to the AFR circuit. Look for a changing voltage in the scan tool’s AFR data PID. The actual amount of change is not important, since this test simply checks for a changing response. This verifies that the circuit is not open or shorted, which would be indicated by NO change on the scanner AFR PID display.

• To perform a current response test of the pump cell circuit, simply break the pump cell circuit wire and connect a digital low-amp meter in between the broken circuit. Start the engine and warm up to operating temperature. Operate the engine at different conditions and compare to table 2. (Table 2 is a general operating current value table, which may differ slightly from one manufacturer to another).


• Perform a voltage reading at the AFR heater power feed wire. Most AFR sensors a fed power through an ECM controlled power relay while the other side of the heater circuit is a pulse modulated to ground. (Determines if the power feed voltage relay is operating properly).

• Connect a low resistance lamp circuit (headlight) to the AFR heater circuit and start the engine. Verify that the lamp turns ON and OFF. Note: The use of a low resistance lamp (headlight) is needed due to the fact that on some systems, the ECM constantly checks the heater resistance. On these systems, if a test light is used, the ECM simply shuts down the heater circuit, since a test light has a resistance of close to 20 Ohms and only draws about 300 mA of current, as opposed to the 8.00 Amps needed by the AFR heater.

• Using a low amperage clamp-on meter, obtain a scope waveform from the AFR’s heater circuit. The waveform should look similar to a pulsating ignition coil, with a series of current humps indicating a duty cycle (pulsing) controlled heater element. The highest part of the waveform should be within 6 to 8 Amps.

• A lack of a current hump on the scope’s display points to the heater circuit not working. Disconnect the AFR sensor and measure the heater continuity. It should be close to 1.5 Ohm since the heater is almost like a straight through wire, which is why it is duty-cycle-controlled. If the heater circuit continuity reading shows an open circuit, replace the AFR sensor. Otherwise, perform a resistance check of the heater circuit wires between the sensor and the ECM/ Power feed circuit.

• A low current reading on the heater circuit indicates a high resistance fault in the heater circuit.

• A higher than 8.00 amps current draw from the heater circuit indicates a shorted heater inside the AFR sensor. Replace the AFR sensor and re-check.


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