Closed Loop

When O2S 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 to control air/fuel ratio based on O2S signals (in addition to other input parameters). ECM maintains air/fuel ratio as close as possible to 14.7:1. If O2S cools off (due to excessive idling) or a fault occurs in the O2S circuit, vehicle once again enters open loop mode.