A/F Sensor: Air Fuel Ratio Sensor

An A/F sensor or air-fuel ratio sensor is different than a conventional O2 sensor. It's considered a wide range or broad range oxygen sensor able to operate under a wide range of operating conditions.

aN air-fuel ratio sensor cutaway.

It provides more accurate monitoring of oxygen content over a broader range than the narrow range oxygen sensor. A narrow range oxygen sensor provides a rich lean cycle in a limited range ~14.6:1 to 1 to 14.8:1 to the ECM.

An A/F sensor provides:

  • A wider range of air-fuel ratios (10:1 to 20:1).
  • A more accurate means of monitoring oxygen content.
  • Changing both voltage and amperage for more precise fuel trim.
  • Lower emissions.
  • Better engine performance.

The A/F sensor offers a full range of fuel ratios as rich as 10:1 to as lean as 20:1. They also run at 1200┬░ F instead of 750┬░ F. They have a planar zirconia element with an ion pump, a resistor, and a heater to quickly reach operating temperature and maintain this temperature while idling or in heavy traffic.

Air-fuel ratio sensor has a wider range of air-fuel ratios.

A rich signal is a negative current with a voltage 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). These sensors are more complex with a larger plug with six and seven wires. Therefore, a scan tool is typically necessary when diagnosing this sensor.

A faulty air-fuel ratio sensor can result in a drop in fuel mileage. The engine management system is making fuel decisions based on this input whenever in closed-loop. On a conventional system, the ECM idly waits for a standard oxygen sensor to come back into range. A newer engine management system with an A/F sensor can monitor the complete range of ratios offering lower emissions and better engine performance.