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Getting Technical - Air Fuel Ratio Sensors

At O2Sensors.com.au we often get asked the difference between an Oxygen Sensor (O2 Sensor) and an Air Fuel Ratio Sensor (AFR). The air fuel ratio sensor is a newer type of oxygen sensor which is slowly replacing the traditional Zirconium oxygen sensor in modern motor vehicles. They started being used on a few Toyota models in 1997, but have been used more and more in recent years. Many Japanese and German cars are the early adopters of this type of sensor.

Both the O2 sensor and AFR sensor monitor the amount of oxygen (O2) in the exhaust gas. The amount of oxygen in the exhaust gas is a good indicator of engine combustion efficiency and is also the best place to monitor the air to fuel ratio.

The task of the Air Fuel Ratio sensor is just that same as that of the Oxygen sensor, the difference is an AFR sensor is not a stand-alone component and works with the AFR sensor specific integrated circuits in ECU to calculate an output signal via small current fluctuations.

Air Fuel Ratio sensors can look the identical to traditional Oxygen sensors but are NOT interchangeable.

air fuel ratio sensors

The AFR sensor provides the ECM (Engine Control Module) with a signal value throughout a wide range (Wide band) of air/fuel ratios the ECM can more accurately measure the actual air/fuel ratio on a wider scale. The wide band sensor not only tells the ECU if the mix is rich or lean, but HOW rich or HOW lean it is.

A detection circuit in the ECM is designed to look at changes in current flow and converts this to a voltage reading. This voltage signal can ONLY be seen by a diagnostic scanner tool through the ECM. It cannot be tested (back probed) with conventional oxygen sensor testers like our ST-05 Sensor Tester and Simulator accessory.

Traditionally diagnosing the health of an oxygen sensor would require a scope connected to the signal wire and measure voltage oscillation to determine the sensors health. Applying this method to an air fuel ratio sensor would reveal a practically flat line of around 3 volts through the whole sensor range, while at the same time reading the voltage on a diagnostic tool would be oscillating. This tells us that even though we have a voltage oscillation like a traditional oxygen sensors, it is a calculated voltage generated by the ECM and not an output voltage from sensor itself.

You will also notice that this voltage moves in the opposite direction to your typical oxygen sensor where typically higher voltage indicates rich and lower voltage indicates a lean mixture, this is opposite with AFR sensors.

AFR sensors are current devices and do not put out an actual voltage for their signal. Air Fuel Ratio sensors communicate their information by providing a current flow, the strength of which is directly proportional to the amount of oxygen present in the exhaust stream, and is very accurate. The amount of current is relatively small (0 - 30 mA).

In this way, it's easier for the ECU to adjust the mix without a lot of guess work. The sensor eliminates the rich/lean cycling inherent in traditional Zirconium O2 sensors, allowing the control unit to adjust the fuel delivery and ignition timing of the engine much more accurately and rapidly.

AFR sensors with their wider range (wide band) are used for greater control of fuelling and mapping thus a faulty unit has greater effect to vehicle performance, emissions, and fuel consumption.

Air fuel ratio sensors also run hotter to achieve a greater range. By doing so they tend to have more heater circuit failures than traditional oxygen sensors. The AFR sensor heater is usually on (pulsing) under normal driving conditions and is pulse-width modulated by the ECM to maintain a stable temperature.

The AFR heater carries more current because of the higher temperatures necessary; the heater circuit carries up to 8 amps compared to the Zirconium O2 sensor at 1.5 to 2 amps. For this reason the connections are more critical so as to avoid resistance in the circuit.

Common problem when fitting an AFR sensor!

Fitting a new AFR sensor is not always plug and play; some vehicles require an ECM reset.

If this is not done correctly the vehicle may run badly as the ECM has previously learnt to compensate for the fault in the original sensor. The new sensors heater may be damaged by too much current being fed to it by the ECM, trying to compensate for the old sensor. An ECM relearn may take significant time and the sensors may be permanently damaged during this time. Pre mature heater failures will void warranty if correct reset procedure has not been carried out.