All You Ever Wanted To Know About Air Fuel Ratio Sensors

It's not a part you think about often, but the air/fuel ratio sensor performs and important duty as a part of your car's emissions system. Here is the 411 on air/fuel ratio sensors.

Understanding Air/Fuel Ratio Sensors

An air/fuel ratio sensor is much like an oxygen sensor, or O2 sensor, and is slowly replacing them on many vehicles. Especially today's more sensitive, turbocharged, and efficient engines. Unlike the vulnerable O2 sensor, an air/fuel ratio sensor is a wide range sensor rather than narrow range, and it operates by conduction rather than generation and is therefore slightly more accurate. Like oxygen sensors, though, air/fuel ratio sensor will be positioned before and after the catalytic converter on OBDII-compliant engines (Sensor 1 is always on the exhaust manifold, Sensor 2 is always after the catalytic converter) and will be referred to as O2 sensors in most of the OBDII literature, despite being air/fuel ratio sensors.

Benefits of Air/Fuel Ratio Sensors:

  • Caters to today's sensitive engines
  • Wide range sensors
  • Better for turbocharged and more efficient engines

Air/Fuel Ratio Sensor vs. O2 Sensor

Emission System at the Catalytic Converter

The two differences in how an air/fuel ratio sensor works versus the older O2 sensor makes all the difference in how they function to help the electronic control module (ECM or ECU) tune the engine's performance.

The older O2 sensors have a central core made of zirconium (or, in rare cases, titanium) which creates voltage when oxygen molecules pass through it. The new air/fuel ratio sensor uses a dual core and dedicated electronic circuitry to control current flow. The dual core acts as a conductor into which the air/fuel ratio sensor's circuitry sends current. The first of the dual cells, called the pump cell or diffusion chamber, receives air from the exhaust stream. Oxygen in that stream determines the conductivity of the cell, creating current differences from that which is being sent by the air/fuel ratio sensor's circuitry and being received back in the circuit. The second cell, the reference cell, houses outside air for reference in the calculations made by the ECM (engine control module).

This allows a broader band of measurement as well as a more precise measure of what's happening in the exhaust stream in real-time. It also allows for faster adjustment. The ECM can change the pulsation-reduction to bring the mixture back to lean or widens injector pulsation to add fuel when the output is rich in oxygen. The air/fuel ratio itself can also have its current adjusted and even reversed in polarity in order to 'suck in' or expel excess oxygen from the Pump Cell to clear it for new measurements.

All of these operations can be done several times per second, allowing for very precise control over the engine's output and emissions. The changes in voltage that the air/fuel ratio sensor measures are minuscule, on the order of milliamps, giving them high precision for measurement, but also making it difficult to test the air/fuel ratio sensor for diagnostics.

Testing must be done using a scanner made for the purpose or a newer OBD scanner that has this functionality. In-line scanning via a meter is not possible with an  air/fuel ratio due to the low changes being measured and the requirement that lines be cut and spliced to make the measurements (which could affect the input/output with added interference). It is necessary to know the normal operating voltage for the vehicle's air/fuel ratio sensor, usually in the volt range, and how your scanner will interpret the results -- which could be in actual amp changes or in lambda ratios (starting at 1.0 as ideal). Higher numbers are lean, lower numbers are rich corresponding to the conduction difference in the Pump Cell.

The Bottom Line on Air/Fuel Ratio Sensors

Manufacturers are moving towards air/fuel ratio sensors because they give a faster, more control-able emissions control option, allowing a more continual optimal air:fuel ratio (usually held at 14.7:1 in a gasoline engine). Where an older O2 sensor may require more than a second to cause adjustments and calibration for air:fuel, an air/fuel ratio sensor allows it to be done in fractions of a second instead, greatly boosting economy.