AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 121)

Page 121 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
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.
Characteristic curve of a Nernst Cell O2 Sensor
The rich region output of narrow band O2 sensors is temperature dependent, which renders it
useless if an accuracy better than 1.5:1 AFR is desired. This is immediately obvious given the
fact that a single output voltage actually represents wildly different air fuel ratios depending on
the unregulated and unmeasured sensor temperature. These sensors were designed for
operating closed loop around stoichiometry (14.64:1 for gasoline), and for performance tuning
they are useless.
The heart of the AEM gauge is the Bosch LSU4.2 Universal Exhaust Gas Oxygen (UEGO)
sensor. This type of sensor is commonly referred to as “laboratory grade” and works on a
different principle than the narrow band oxygen sensor found in most vehicles. Its unique design
makes precision AFR measurements possible over the entire operating range.
UEGO sensors use a “current pump” to determine the actual oxygen concentration within the
sensing element or. The output is in the form of a very small current, which varies depending on
the air-fuel ratio. This is completely different from a narrow band oxygen sensor, which directly
outputs a voltage. The UEGO sensor design allows measurement of the exact air fuel ratio over
the entire operating range.
The following diagram illustrates the simple installation of an AEM UEGO gauge controller kit.