A/F Sensor: Air Fuel Ratio Sensor
An A/F sensor or air-fuel ratio sensor is different than a conventional O2 sensor found on an engine's exhaust system. It's considered a wide range or broad range oxygen sensor. It provides a more accurate means of monitoring oxygen content than the narrow range O2 sensor. A narrow range oxygen sensor provides a rich lean cycle to the PCM.
An A/F sensor provides a signal in proportion to oxygen content, changing both voltage and amperage for more accurate fuel trim and engine performance. They also run at 1200° F instead of 750° F. A heater is added to reach operating temperature and maintain this temperature during extended idle periods, such as heavy traffic.
The A/F sensor provides a range of fuel ratios as rich as 12:1 to as lean as 19:1. A rich signal is a negative current with a voltage signal below 3.3 V. A lean signal is a positive current with a voltage above 3.3 volts. If there is no change in current and voltage is at 3.3 V, it is at stoichiometry (14.7:1). A scan tool is necessary when diagnosing this sensor.