What is the difference between a wide band and narrow band O2 sensor?

A narrow band O2 sensor can only correctly measure the stoichometric air fuel ratio. For gas this ratio is 14.7 to 1, but this air fuel ratio is not ideal for all applications. Since the ability to measure a specific air fuel ratio other than 14.7 to 1 accurately with a narrow band O2 sensor is not possible, a wide band O2 sensor is employed. This type of sensor can accurately measure air fuel ratios from 8 to 1 up to 16 to 1, and is preferred over the narrow band. Both ECUs offered use a wide band O2 sensor.