Air-Fuel Ratio (AFR) Sensors are alternatives to oxygen sensors (O2 sensors) in modern motor vehicles. Air-Fuel Ratio Sensors are more technologically advanced with extra exceptional internal circuitry for precise air/fuel ratio of the engine. Modern vehicles and power-train engine control modules (PCM) using Air-fuel ratio (AFR) sensors instead of oxygen sensors. AFR sensor measures the air/fuel ratio with reference to 14.7:1, and generates current if the air/fuel mixture is either rich or lean mixture.

Because lowering exhaust emissions and increasing fuel economy continues to be the aim of import manufacturers, a much wider range of air/fuel ratios must now be measured with Air-fuel ratio (AFR) sensors than is possible with the conventional oxygen sensor. Both, however, generate a similar signal for the power-train control module (PCM) to process.

By definition, the Air-fuel ratio (AFR) or Lambda is the mass relation of air and fuel that are present in the combustion process. Once all the fuel mix with all the free oxygen in the combustion chamber, the chemical balance is resulted for AFR, which is called the stoichiometric mixture. It is very necessary to monitor the AFR due to the anti-pollution and vehicle performance tuning purposes. The stoichiometric mixture is the working point where modern engine management systems having the fuel injection. Most fuels have the combination of octane, heptane, and some alkanes, with a couple of detergents and oxygenators. Such compounds can change the stoichiometric ratio, as majority of the additives can force down the ratio downward. This happens since the oxygenators provide excess oxygen to the combustion.

With Air-fuel ratio (AFR) sensors, the engine’s PCM measures air/fuel ratios by monitoring a very small current that flows through the AFR sensor. A stoichiometric or chemically correct air/fuel ratio of 14.7:1 generates zero current flow through the AFR sensor. If the mixture is rich, a negative current flow is returned to the PCM. A lean mixture returns a positive current flow to the PCM.

Zirconia oxygen sensors and Titanium oxygen sensors are two types of oxygen sensors. Zirconium sensor and Titanium oxygen sensors are narrow-band oxygen sensors. The main problem with any narrow-band O2 sensors is that the Engine Control Module (ECM) only detects that the mixture is slightly richer or leaner than 14.7:1. AFR sensor is a wide-band oxygen sensor with high sensitivity to put the ECM in a better position to control the air/fuel mixture.

Another major difference between the wide-band AFR sensor and a Zirconium O2 sensor is that it has an operating temperature above 1200°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 control the heater temperature precisely. The ECM controls the heater circuit.