An air–fuel ratio meter monitors the air–fuel ratio of an internal combustion engine. Also called air–fuel ratio gauge, air–fuel meter, or air–fuel gauge. It reads the voltage output of an oxygen sensor, sometimes also called lambda sensor, whether it be from a narrow band or wide band oxygen sensor.
The original narrow-band oxygen sensors became factory installed standard in the late 1970s and early 1980s. In recent years, a newer and much more accurate wide-band sensor, though more expensive, has become available.
Most stand-alone narrow-band meters have 10 LEDs and some have more. Also common, narrow band meters in round housings with the standard mounting 2 1/16″ and 2 5/8″ diameters, as other types of car ‘gauges’. These usually have 10 or 20 LEDs. Analogue ‘needle’ style gauges are also available.
As stated above, there are wide-band meters that stand alone or are mounted in housings. Nearly all of these show the air–fuel ratio on a numeric display, since the wide-band sensors provide a much more accurate reading. And since they use more accurate electronics, these meters are more expensive.