AEM 30-4110 Digital Wideband UEGO Gauge - Manual (Page 3)

Page 3
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. (Figure 1)
Figure 1. 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 LSU4.9 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. 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.