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Bad O2 Sensor Symptoms

Bad O2 Sensor Symptoms

What function does an O2 sensor serve? What are the symptoms indicating a malfunctioning sensor? Read on, to find all the answers.
Omkar Phatak
The greatest contributing factors to air pollution are the emissions from our gas-guzzling automobiles. Designers have worked hard to develop mechanisms that can reduce emission of harmful gases into the atmosphere. One of the most effective of these mechanisms is the oxygen (O2) sensor, which facilitates feedback based control.


An oxygen sensor is a part of the Engine Control Unit's (ECU) sensor mechanism, which enables it to modify the air-fuel mixture ratio, according to the RPM of the engine as the vehicle moves on the road. The oxygen sensor indirectly measure how much oxygen is required for optimum performance of the engine. It is fitted in the exhaust and is an electronic device, which sends varying voltage signals to the ECU, which enables emission control.

The Symptoms

Dealing with sensor problems is essential for the well-being of your car's engine. If you can recognize signs of malfunctioning early, you can prevent a lot of fuel loss and pollution that may result from incomplete combustion. Here are the prime signs of a bad oxygen sensor.

Increased Emission of Polluting Gases
With malfunctioning of the sensor, the engine control unit can no longer control the throttle sensibly, resulting into uncontrolled engine functioning. This means that the air-fuel mixture will no longer be controlled according to engine RPM demand. So the fuel won't be utilized optimally, resulting into a lot of uncontrolled emissions of polluting gases. This can be prevented, if you can only get the sensor replaced, as soon as possible.

Substantial Drop in Gas Mileage
One of the most visible signs is a sudden drop in your car's mileage. The efficiency of the engine takes a hit, when the sensor stops functioning. That's why, it is necessary that you get it checked when you notice a pronounced drop in the mileage.

Engine Noise
One of the more ambiguous symptoms to recognize is engine noise, as it can be caused by several other factors. Due to lack of air-fuel mixture control, it may so happen, that too lean a mixture may get created in the engine, causing pre-ignition. This may lead to a loud knocking noise. Of the several things that may cause it, O2 sensor is one. Of course, replacing it is important if it has stopped functioning.

Irregular Engine Functioning
One of the other symptoms is the irregular functioning of the engine, caused by the cycle timing going haywire. This may happen as the engine control unit is blinded by the lack of feedback about air-fuel ratio, which is partially supplied by the sensor.

By acting promptly, you can save some precious fuel, which will compensate for the replacement cost and also prevent the high level of emissions caused by the oxygen sensor malfunction. Make it a point, that you include sensor testing and maintenance in your periodic car maintenance schedule. This will go a long way in maintaining the efficiency of your car engine and will substantially reduce the pollution caused by your automobile.