Closed Loop

When O2 sensor has reached operating temperature, coolant temperature has reached a preset temperature and a specific period of time has passed since engine start-up, ECM operates in "closed loop". In "closed loop", ECM controls air/fuel ratio based upon O2 sensor signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel mixture as possible. If O2 sensor cools off (due to excessive idling) or a fault occurs in the O2 sensor circuit, vehicle will once again enter "open loop" mode.