Closed Loop

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