Heated Oxygen Sensor
Oxygen sensors are located in the exhaust manifold or exhaust pipe of an automobiles exhaust system. The sensor sends an analog voltage signal to the PCM that varies between 0 and 1 volt. The PCM uses this information to control air fuel ratio.
This sensor produces a reliable voltage signal after it reaches the temperature of 600° F. This is due to the design and materials used to create the oxygen sensor. The most common, the zirconia sensing element is a conductor of voltage when it is hot and a resistor when cold. As oxygen passes through the element it effects the voltage signal. It will produce a high voltage signal when the engine is running rich and a low signal when the engine is running lean.
When the oxygen sensor reaches 600° F it is in closed loop. In a closed loop system the PCM attempts to maintain a stoichiometric air to fuel ratio of 14.7:1. This is the best air to fuel ratio during idle and cruising speeds. The computer makes a small adjustment and then checks the result with the sensors voltage signal. This results in a cycle of a slightly rich adjustment followed by a slightly lean adjustment. While cold, at wide open throttle, and during high load conditions the system goes into open loop. This means that the signal is ignored and the PCM will enrich the mixture to accommodate immediate demand.
The sensor can have one to four wires. A one wire and two wire sensor does not contain a heater circuit. This means that the PCM must wait for this signal until the sensor warms up to operating temperature. Three and four wire O2 sensors have a heater that brings the sensor to operating temperature sooner. The only difference is that the three wire sensor shares a common ground between the heater and sending circuits. Because it reaches operating temperature sooner, the heated oxygen sensor is more efficient than the unheated sensor.
