O2 (oxygen sensor) – AEM 30-71XX Infinity Stand-Alone Programmable Engine Mangement System Full Manual User Manual
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© 2014 AEM Performance Electronics
Infinity User Manual
O2 (Oxygen Sensor)
There are many types of O2 sensors that are employed by vehicle manufacturers, and it is well
beyond the scope of this manual to describe all of them. An O2 sensor provides a reading of the
air/fuel ratio (AFR) to the ECU so that it can make the necessary fuel calibration corrections to
achieve a desired Air Fuel Ratio (AFR).
An O2 sensor works by sensing whether there is an abundance or lack of oxygen in the exhaust
gases, depending on whether the gas mixture is too rich or too lean. If there is excess oxygen
and the mixture is too lean, output voltage from the O2 sensor to the ECU will be high. The ECU
may then compensate by adding fuel. The converse is true of rich mixtures.
Common O2 sensors include 3, 4, and 5-wire heated or wide-band sensors. Three- and 4-wire
sensors are ideal for determining whether a vehicle’s AFR is at the optimum stoichiometric ratio.
Stoichiometric ratio refers to the ideal mixture of fuel and air by mass to completely consume
both reactants (gas and air) with nothing left over. Based on the properties of most pump
gasoline used today this ratio is typically a 14.64:1 air/fuel ratio. Although this ratio provides the
best combustion characteristics with the least emissions output and optimum catalytic converter
performance, it is NOT the best AFR for maximum power at full throttle or under boost. This
mixture is too lean and may cause engine damage.
Typical production vehicle oxygen sensors rely on “Nernst Cell” technology, commonly called
“Narrow Band” and sometimes erroneously described as “Wide Band.” This is a very cost
effective method that outputs a voltage based on the oxygen content of the gas being sampled. It
is accurate in the region surrounding stoichiometric operation and leaner. Unfortunately, in the
rich region where high performance engines usually operate, their accuracy and repeatability is
virtually non-existent.