Closed Loop

When O2 sensor reaches operating temperature, coolant temperature reaches 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 ratio as possible. If O2 sensor cools off (due to excessive idling) or a fault occurs in the O2 sensor circuit, vehicle once again enters open loop mode.