How does O2 sensor feedback operate in closed-loop fueling?

• A. The ECU ignores O2 readings
• B. The ECU compares actual AFR to target and adjusts injector pulse width
• C. The ECU only adjusts ignition timing
• D. The ECU reduces fueling

Answer: (B)

Closed-loop fueling relies on the O2 sensor to continually balance the air-fuel mixture. The sensor reports whether the exhaust is richer or leaner than the desired target. The ECU has a target AFR (often around the stoichiometric ratio for gasoline) and uses the sensor signal to compare the actual AFR to that target. When the exhaust is too rich, the ECU reduces the injector pulse width to cut fuel; when it’s too lean, it increases the pulse width to add fuel. This keeps the engine’s fueling tracking the target over different loads and temperatures, with short-term adjustments plus long-term trims learned over time. Ignition timing adjustments aren’t the primary mechanism for maintaining the target mixture in this closed-loop process, and simply reducing fueling without feedback wouldn’t reliably reach the target.