Oxygen Sensor
WARNING: This page is about a different car, the 1984 Volvo 760, 1984 Volvo 245, 1984 Volvo 244, and 1984 Volvo 242. However, it is still accessible from the selected car via links, so may be relevant.
The oxygen sensor is located in the exhaust manifold and measures the amount of unburned oxygen in the exhaust gas. If oxygen is low (rich mixture) a high voltage will be generated by the sensor. If oxygen is high (lean mixture) low voltage will be generated. The voltage signal from the oxygen sensor is sent to an electronic control unit which controls fuel mixture.