oxygen sensor

Introduction
The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the exhaust as it exits the engine. Monitoring oxygen levels in the exhaust is a way it gauges the fuel mixture. The sensor tells the ECU if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).

The O2 sensor works like a miniature generator and produces its own voltage when it gets hot. Inside the vented cover on the end of the sensor that screws into the exhaust manifold is a zirconium ceramic bulb. The bulb is caoted on the outside with a porous layer of platinum.Inside the bulb are two strips of platinum that serve as electrodes or contacts.Between the electrodes is a solid-state electrolyte made up of a zirconic ceramic material that acts like a galvanic battery electrolyte under certain conditions. When the sensing element is cold, the zirconia material behaves similar to an insulator . At elevated temperatures, the zirconia material performs more like a semiconductor, and can generate a characteristic voltage output on the sensor connections. The outside of the bulb is exposed to the hot gases in the exhaust while the inside of the bulb is vented internally through the sensor body to the outside atmosphere. Newer-style O2 sensors breathe through their wire connectors and have no vent hole. This is why grease should never be used on O2 sensor connectors because it can block the flow of air.