Malfunctioning oxygen (O2) sensors are the most common cause behind a check engine light, but what exactly does the sensor do and how does it break down? Oxygen sensors have been a required part of nearly every vehicle's construction since 1981. They are designed to keep engine performance and efficiency at peak levels at all times while also keeping emissions in check.

The O2 sensor is part of the emission control system stationed in the exhaust pipe, and monitors the gases leaving the engine to ensure the correct amount of oxygen is reaching the combustion chamber. A gasoline-powered engine relies on a precise ratio of air and fuel to burn efficiently, and following combustion, the sensor detects how much unburned oxygen is present in the exhaust gases to determine if the mixture was too rich or too lean and adjust accordingly.

The relay of information from the sensor to the computer is referred to as a closed loop. When there is too much air in the mixture, it is too lean, and too little air leads to a rich mixture. When the O2 sensor reports a lean mixture, the computer alters the ratio to allow more fuel. This changes the ratio to be more rich, and the sensor will indicate the switch, causing the ratio to change back again. This constant fluctuation actually helps the catalytic converter operate most efficiently and reduce emissions as much as possible.

When the oxygen sensor acts up, the car's engine management computer is forced to guess at the appropriate mixture ratio, which can lead to inefficiency and loss of performance. If the sensor fails completely and no readings are received, the computer sets a fixed ratio, which allows the engine to continue working. However, the unchanging levels of air can dramatically affect the emissions and fuel economy of the vehicle. When the O2 sensors begin to fail, they should be dealt with as soon as possible.

Aside from the check engine light, which could point to a variety of problems, there are a few other signs the oxygen sensors are failing. High emissions and worsening fuel economy are obvious signs, but you may not notice if your car is using a fraction more gas than usual, and it's even harder to tell your vehicle's emission levels unless you're having the vehicle inspected.

It is far easier to spot a failing sensor by paying attention to the engine's condition. The faulty sensor can send inaccurate readings to the computer that disrupt the engine timing and combustion intervals as well as unbalance the fuel-to-air ratio. This may cause the motor to idle roughly.  The engine might also miss while idling or operating at parking lot speeds. When the timing is thrown off, the sensor may cause engine pinging. Pinging, or engine knock, is a metallic sound that occurs in the engine when the combustion process is disrupted and thrown out of its proper cycle. Typically, pinging occurs when the fuel mixture in the combustion chamber is ignited at the wrong time.

The sensing mechanism in the O2 sensor may have worn out naturally, and the rule of thumb for replacing this part is to do so every 60,000 to 100,000 miles. However, this can vary depending on the car and the sensor, so consult your owner's manual for a precise schedule to maintain the part.