The result is a constant flip-flop back and forth from rich to lean, which allows the catalytic converter to operate at peak efficiency while keeping the average overall fuel mixture in proper balance to minimize emissions. This is referred to as ‘closed loop’ operation because the computer is using the O2 sensor’s input to regulate the fuel mixture. This produces a corresponding change in the O2 sensor reading. Carley explains it further: “The computer uses the oxygen sensor input to regulate the fuel mixture, which is referred to as the fuel ‘feedback control loop.’ The computer takes its cues from the O2 sensor and responds by changing the fuel mixture.
Our second definition is “closed loop.” The oxygen sensor at this point is now being used for feedback control. This is referred to as ‘open loop’ operation because no input is used from the O2 sensor to regulate the fuel mixture.” According to Babcox Technical Editor Larry Carley, “when no signal is received from the O2 sensor, as is the case when a cold engine is first started (or the O2 sensor fails), the computer orders a fixed (unchanging) rich fuel mixture. The first definition is “open loop.” Simply stated, the oxygen sensor is not being used for feedback control. We’ll begin our discussion by looking at each one on an individual basis. This month, we will explore the new fuel system definitions that are present on many of today’s vehicles. It’s become very important to extend our understanding well beyond the concepts of simple “open loop” and “closed loop” status. Many modern-day fuel systems offer a new look into system status definitions. There are many symbols and definitions of status yachts, luxury cars, diamonds but if you’re reading this, right now you’re probably more concerned with “system status” definitions.