Open Loop Mode
WARNING: This page is about a different car, the 2003 GMC Savana and 2003 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.
When engine is cold and engine speed is greater than 400 RPM, PCM operates in open loop mode. In open loop mode, PCM calculates air/fuel ratio based upon coolant temperature and MAP sensor readings. Engine remains in open loop mode until oxygen sensor reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has elapsed after engine starts.