The oxygen sensor (O2S) in your Honda Civic is part of the engine emission control system, which helps reduce the amount of poisonous gases released into the atmosphere. Monitoring oxygen content in exhaust gases is an excellent way to analyse an engine operating condition. And this is accomplished through the use of the oxygen sensor.
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If you open the bonnet of your Honda Civic, most likely you will locate the oxygen sensor (O2) mounted in the exhaust manifold, above the exhaust pipe flange. On some models the exact location might vary, but you can always find the sensor by following the exhaust manifold and pipe. Depending on the year model of your Honda Civic, you may find a Primary Oxygen Sensor (PHO2) right before the catalytic converter and a Secondary Oxygen Sensor (SHO2) right after the catalytic converter.
When your Honda Civic reaches operating temperature, the O2 sensor starts to send a voltage signal-between 100 and 900 milivolts (mV)-to the engine control computer that corresponds to the amount of oxygen present in the exhaust gas stream passing through the catalytic converter. A high concentration of oxygen detected-closer to 100 mV-is interpreted by the computer as a lean fuel-air mixture. The engine computer then makes the adjustments to increase fuel injection. As the sensor detects lower oxygen content and sends the corresponding signal--closer to 900 mV--the computer adjusts and decreases fuel injection. And the cycle begins again to maintain fuel injection within predetermined parameters stored in the computer memory.
Most oxygen sensors use zirconia and platinum as the active elements to detect the presence of oxygen in exhaust gases and produce a voltage signal. Also, zirconia oxygen sensors may use a heating element--Heated Oxygen Sensor (HO2S)--that allows the component to start operating even before the engine reaches its own operating temperature, improving driveability and fuel economy. Titania oxygen sensors, introduced later, modify a voltage signal from the computer through resistance to monitor oxygen content.
Up to the fifth generation (1972-1995), all Honda Civics came equipped with only one oxygen sensor. In 1996, a second generation of vehicle On-Board Diagnostic (OBD-II) computers were introduced in the United States and the use of two oxygen sensors was required. The Primary Oxygen (PHO2) sensor is placed between the catalytic converter and the exhaust system. The secondary sensor, located after the catalytic converter, is now used to monitor the first sensor to improve fuel economy and emissions control.
Working on the exhaust system of any vehicle to troubleshoot or replace an oxygen sensor may require the engine to be brought up to operating temperature. Take extreme caution because the exhaust system may reach temperatures of 1,500F (816 C) or more and could cause severe burns to the skin.