Which sensor monitors the air/fuel ratio before the catalytic converter?

The sensor that monitors the air/fuel ratio before the catalytic converter is the upstream oxygen sensor. This sensor is positioned upstream of the catalytic converter in the exhaust system and is primarily responsible for measuring the amount of oxygen in the exhaust gases.

By assessing the air/fuel ratio, the upstream oxygen sensor provides vital data to the engine control unit (ECU), which then adjusts the fuel injection to optimize combustion efficiency and reduce emissions. This is crucial for maintaining vehicle performance and ensuring the catalytic converter operates effectively in reducing harmful exhaust gases.

In contrast, other sensors have distinct roles. The downstream oxygen sensor measures the exhaust gases after they have passed through the catalytic converter, providing feedback on its efficiency. The mass air flow sensor estimates the amount of air entering the engine to help calculate the right fuel mix, while the fuel pressure sensor monitors the fuel pressure in the system but does not directly measure the air/fuel ratio.