EGT: The Exhaust gas temperature sensor is the easiest to explain. This is a sensor usually located near to the front of the exhaust and it measures the temperature of the exhaust gasses as they pass through the exhaust.
At a balanced air-fuel ratio, the exhaust gas temperature is lower than that in a lean or rich air-fuel ratio. The ECU uses this data to adjust the fuelling. This meter is most used in tuning turbo-equipped cars. If the sensor is installed at the manifold collector or up pipe before the turbo, the turbine inlet temperature can be monitored. If the sensor is installed after the turbo, the exhaust temperature can be monitored. Because EGT sensor will typically drop 200-300 degrees F across the turbine, installers in general will try to put the thermocouple as close to the cylinder head as possible to give a true reading that will require less mental math to read properly, and a reading that will react faster to the engine's condition compared to an installation after the turbo. EGT alone is considered an older technique for getting the most out of an engine.
AFR: Air/Fuel ratio sensor. This sensor is installed in the exhaust before the catalytic converter and is often called a precat O2 sensor. It compares the O2 content of the exhaust and that of the surrounding ambient air. The theoretically best mixture ratio for gasoline engines is when all of the fuel will react with all of the available oxygen resulting in complete combustion of the fuel and air.
This is called The stoichiometric (stoy kee o met ric) air-fuel ratio.
O2 Sensor (Lambda Sensor/Oxygen Sensor): This is installed in the exhaust after the catalytic converter and measures the amount of oxygen (O2) in the exhaust gases after the cat and works with the AFR sensor. The outside of the bulb is exposed to the hot exhaust gases, while the inside of the bulb is vented internally through the sensor body or wiring to the outside atmosphere. When the air/fuel mixture is rich and there is little O2 in the exhaust, the difference in oxygen levels across the sensing element generates a voltage through the sensor's electrodes.
The oxygen sensor's voltage signal is monitored by the onboard engine management computer to regulate the fuel mixture. When the computer sees a rich signal (high voltage) from the O2 sensor, it commands the fuel mixture to go lean. When the computer receives a lean signal (low voltage) from the O2 sensor, it commands the fuel mixture to go rich. Cycling back and forth from rich to lean averages out the overall air/fuel mixture to minimize emissions and to help the catalytic converter operate at peak efficiency (which is necessary to reduce hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) levels even further).
O2 and AFR sensors do not last forever. The life expectancy is between 60000-100000miles.
Keeping the sensor fresh may improve fuel economy as much as 10-15% (which can save $100 each year in fuel costs on the average). Keeping the sensor in good operating condition will also minimize exhaust emissions, reduce the risk of costly damage to the catalytic converter, and ensure peak engine performance (no surging or hesitating).
For these reasons, the O2 sensor should be considered a "tune-up" replacement item just like spark plugs, especially on older vehicles (those built before the mid-1990's) where the life expectancy is only around 30000 miles..
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