A critical component of today's modern vehicle is the sensor that measures the amount of oxygen in the exhaust gases. This sensor works with the vehicle's Electronic Control Unit (ECU) to constantly maintain the operational efficiency of the engine. If the oxygen content in the exhaust gas is too high (a lean mixture) or too low (a rich mixture), the sensor is able to transmit this information back to the ECU, which in turn adjusts the amount of fuel to be sent to the cylinder in order to maintain the perfect air to fuel ratio.
Not all vehicles have sensors with the same output characteristics and these sensors are not interchangeable. It is important to confirm the sensor type before making the repair.
In vehicles equipped with a narrow-band oxygen sensor, the output voltage changes in accordance with the oxygen concentration in the exhaust gas. The ECU uses this output voltage to determine whether the present air/fuel ratio is richer or leaner than the stoichiometric air/fuel ratio (14.7:1).
Vehicles equipped with an air-fuel ratio sensor have approximately 0.4V constantly applied to the sensor, which outputs a current that varies in accordance with the oxygen concentration in the exhaust gas. The ECU converts the changes in the output current into voltage allowing a response that is directly proportional to the input of the present air/fuel ratio in the exhaust system.
Differences between an oxygen sensor and an air-fuel sensor:
The exterior appearance of the air-fuel ratio sensor and oxygen sensor may be very similar but that's where the similarity stops.
a) Using a light bulb analogy, the oxygen sensor sends voltage to the vehicle's ECU and acts like an on/off switch turning the bulb on and off. The air-fuel ratio sensor receives voltage from the vehicle's ECU and acts like a dimmer switch making the bulb brighten and dim.
b) A narrow-band oxygen sensor determines whether the engine is running rich or lean without measuring the extent of richness or leanness. A wide-band oxygen sensor or the air-fuel ratio sensor is efficient in determining how much rich or lean the engine is running at any given time. This makes the air-fuel ratio sensor preferable over oxygen sensors in modern automobiles.
Symptoms of a defective Oxygen/Air-Fuel Ratio Sensor:
Since both the sensors basically measure the amount of oxygen in the exhaust, they exhibit similar symptoms when they go bad. Common indications of a bad oxygen/air-fuel ratio sensor include rough idling, engine pinging, poor gas mileage and increased exhaust emissions. One of the first symptoms of a faulty sensor is the lighting up of the "Check Engine" light.
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