In EFI engine control, which operation uses oxygen sensor feedback to maintain the target air-fuel ratio?

• A. Open-loop operation
• B. Closed-loop operation
• C. Idle operation
• D. Prime/startup operation

Answer: (B)

Closed-loop operation is the mode where the engine control unit uses oxygen sensor feedback to keep the air-fuel ratio at the target value. The oxygen sensor in the exhaust continuously reports how rich or lean the mixture is. The ECU compares that real-time signal to the desired target (usually around the stoichiometric ratio for gasoline engines) and makes small adjustments to fuel delivery to correct any deviation. If the sensor shows lean conditions, the system increases fuel slightly; if it shows rich conditions, it reduces fuel. This ongoing feedback keeps the mixture near the target, improving combustion efficiency and emissions. In contrast, open-loop operation uses a fixed fuel map without sensor feedback, and startup or idle modes may temporarily run in open-loop until sensors reach operating temperature or conditions allow closed-loop control.