WHY WOULD YOU want to monitor the air/fuel ratio as you drive? Well, for starters, it will allow you to save fuel since the display clearly indicates when the engine is running rich.

When used in conjunction with a wideband oxygen sensor and controller, the air/fuel ratios shown on this unit are more accurate than can be obtained from the narrowband sensors that are typically used in cars and which are really only accurate close to the "stoichiometric" point (ie, the air/fuel ratio at which there is just enough oxygen in the air to ensure complete combustion).

Fig.1: here's how the display unit is used with a wideband sensor and its associated controller. The narrowband S-curve output from the controller is fed to the engine management computer (see text).

Under normal driving, most engine management systems operate under "closed loop" control. This is where the air/fuel ratio from an oxygen sensor is monitored and controlled by the car’s engine control unit (ECU) to maintain a predetermined fuel mixture. This is usually stoichiometric but under light cruise conditions the mixture can go
lean to improve fuel economy.

Conversely, during acceleration, the air/fuel mixture in many cars is allowed to go rich to improve performance and is not under the control of the ECU. This is called "open loop" and the richness of the mixture depends on other factors such as the throttle setting and the injector opening period.