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 the 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 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 of narrow range sensors. A heater is added to reach operating temperature and maintain this temperature during long idle periods, such as heavy traffic.
The A/F sensor changes current and voltage to provide a range of fuel mixtures as rich as 12:1 to as lean as 19:1. When a rich signal is sent to the PCM, it is a negative current with a voltage signal below 3.3 V. When a lean signal is sent to the PCM, It’s 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. A scan tool is necessary when diagnosing this sensor. The signal will typically be steady around 3.3 V.