The first oxygen sensor is located in the exhaust manifold. This sensor is responsible for detecting the amount of oxygen in the exhaust gas before it enters the catalytic converter.
The second oxygen sensor is located in the exhaust pipe, after the catalytic converter. This sensor is responsible for detecting the amount of oxygen in the exhaust gas after it has passed through the catalytic converter.
The oxygen sensors are used by the engine control computer (ECU) to adjust the air-fuel mixture. The ECU uses the input from the oxygen sensors to determine how much fuel is needed to be mixed with the air to achieve the ideal combustion conditions.
