Congratulations! The 52mm (2-1/16”) AEM Universal Exhaust Gas Oxygen (UEGO)
Gauge features a digital readout and sweeping 24 color-coded light emitting diode
(LED) display, providing immediate reference to the engine air fuel ratio (or lambda) in
real-time. The AEM gauge is ideal for all vehicles including carbureted applications and
engine dynamometers. A user-programmable 0-5V analog output is included and can
be used with data loggers as well as most Electronic Fuel Injection (EFI) systems
including the AEM Engine Management System (EMS). A serial data stream is also
integrated for air fuel (or lambda) ratio output to a RS-232 com port.
Because the AEM gauge utilizes the internal AEM UEGO controller and Bosch UEGO
Sensor, it is accurate and repeatable to 0.1 of an air/fuel ratio point! With this, there is
no abrupt oscillation as found in many competitor gauges, which utilize a narrow band
oxygen sensor detecting only stoichiometry.
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 dependant, 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
