View source for Air Fuel Ratio(Measured)

Air Fuel Ratio uses the O2 sensors in the vehicle to measure the mixture of fuel to air burned.

The O2 sensors are not immediately useable on most vehicles as soon as the engine has started, they need time to warm up to a few hundered degrees C before they can be used. During this time, if your ECU supports this sensor, it will be unavailable and your engine will be running in what is known as 'Open Loop' mode

After a short time has passed, the sensor will be available, and the engine will switch to 'Closed Loop' mode (where the timing and fuel calculations are then based upon the readings from various sensors).