"Open Loop"

"Open Loop"
When the engine is first started, and it is above 400 RPM, the system goes into "Open Loop" operation. In "Open Loop," the control module ignores the signal from the HO2S, and calculates the air/fuel ratio based on inputs from the Engine Coolant Temperature (ECT) sensor and Manifold Absolute Pressure (MAP) sensor.
The system stays in "Open Loop" until the following conditions are met.
1. The HO2S has varying voltage output, showing that it is hot enough to operate properly. (This depends on engine temperature.)
2. The Engine Coolant Temperature (ECT) sensor is above a specified temperature.
3. A specific amount of time has elapsed after starting the engine.
A normal functioning system may go into "Open Loop" at idle if Heated Oxygen Sensor (HO2S) temperature drops below the minimum requirement to produce voltage fluctuation.