The Oxygen Sensor (O2 sensor) is mounted in the exhaust manifold or pipe before the catalytic converter. Its job is to measure the amount of unburned oxygen in the exhaust gas as it exits the engine. This data tells the Engine Control Unit (ECU) if the fuel mixture is too lean (too much air) or too rich (too much fuel). The ECU then adjusts the fuel injection duration in real-time to ensure optimal combustion efficiency and lower emissions. A bad O2 sensor causes poor fuel economy, rough idling, and is a leading cause of “Check Engine” lights.