Which arrangement describes the typical O2 sensor placement relative to the catalytic converter?

• A. Located only upstream.
• B. Located in the exhaust manifold.
• C. Located only downstream.
• D. Located both upstream and downstream of the catalytic converter.

Answer: (B)

Oxygen sensors are used to monitor the exhaust before any treatment by the catalytic converter so the engine can adjust the air–fuel mix in real time. The typical upstream sensor sits in the exhaust manifold because it samples unburned gases right after combustion, providing the ECU with the timely signal needed to optimize fueling. While many systems also have a second sensor after the catalytic converter to check catalyst efficiency, the question focuses on the common placement for the sensor that drives fuel control, which is in the exhaust manifold (upstream). This location is favored for fast response and direct measurement of the engine’s combustion output before any catalyst treatment. Choosing a placement downstream would describe catalyst monitoring rather than the primary feedback for fuel adjustment, and describing it as only upstream or only downstream misses the most generally recognized up-stream position used for control.