Closed Loop

When HO2S reaches operating temperature, coolant temperature reaches a preset temperature, and a specific period of time has passed since engine start-up, PCM operates in closed loop to control air/fuel ratio based on HO2S signals (in addition to other input parameters). PCM maintains air/fuel ratio as close as possible to 14.7:1. If HO2S cools down (due to excessive idling) or a fault occurs in the HO2S circuit, vehicle once again enters open loop mode.