If your Check Engine Light suddenly turns on, there is a good chance you have a problem with one of your vehicle’s oxygen sensors. Faulty O2 sensors are a common cause of illuminated Check Engine Lights. Still, because the light may indicate a variety of other problems, it is important to watch for bad O2 sensor symptoms. If you notice any, repairing the problem as quickly as possible is a good idea to prevent further trouble.

Purpose of Oxygen Sensors

Most vehicles on the road today have internal combustion engines. To generate the power necessary to move the vehicle, the engine burns a precise mixture of fuel and air. This process, of course, creates certain emissions. Both to improve engine efficiency and minimize emissions, vehicles manufactured after 1980 have at least one O2 sensor.

With few exceptions, automotive internal-combustion engines require a oxygen-to-gasoline ratio of 14.7:1. If there is not enough oxygen in the mixture, excess fuel sticks around after combustion. Mechanics call this a rich mixture. If the reverse happens, the phenomenon is known as a lean mixture. Both can wreak havoc on your vehicle’s performance and gas mileage. Even worse, the wrong fuel mixture can result in serious engine damage.

Car manufacturers place oxygen sensors in specific areas to ensure the fuel mixture is neither rich nor lean. The number of sensors in a vehicle, though, differs. Those with V6 and V8 engines typically have three. A car with a four-cylinder engine, on the other hand, may more commonly only have two.

Operationally, O2 sensors are basic. If the fuel-to-oxygen ratio is off-balance, a chemical reaction creates voltage. This voltage sends information to the vehicle’s electronic control module. Thus, if the sensor is not working properly, the computer must guess the amount of fuel that is necessary. If the guess is incorrect, pollution and poor performance may be the result.

Signs of a Faulty or Failing Oxygen Sensor

An oxygen sensor does not have to fail completely to give you problems. On the contrary, even a failing O2 sensor (sometimes called a “lazy” O2 sensor) may cause your car to have a few issues. If you notice any of the following signs, you may have a bad O2 sensor or one that is about to go bad:

  • An illuminated check-engine light
  • A rough engine idle
  • A misfiring engine
  • Worsening fuel economy
  • Failed emissions inspection

Even if your car’s engine is not exhibiting any of the above oxygen sensor symptoms, it may still be time to replace your existing ones. In simple terms, O2 sensors are designed to wear out over time. As such, if your vehicle has driven 60,000 to 100,000 miles on with its existing sensors, depending on make and model, you may want to consider replacing its O2 sensors before they fail. Your vehicle’s owner’s manual may specify a service interval you better understand when to preventatively install new sensors.

The following parameters may be helpful guideline for when to replace your oxygen sensors:

  • Model years from the mid-1990s to present day, replace O2 sensors every 100,000 miles
  • Model years from the mid-1980s to mid-1990s with heated three- and four-wire sensors, replace O2 sensors every 60,000 to 70,000 miles
  • Model years from the mid-1970s to early 1990s with unheated one- and two-wire sensors, replace O2 sensors every 40,000 to 50,000 miles

Diagnostic Procedures

If you suspect your car has an old, damaged or ineffective oxygen sensor, you can likely either confirm or dispel your suspicions with an OBD-II scanner. These scanners plug into your vehicle’s computer and display diagnostic trouble codes. The exact error code for identifying a potentially faulty oxygen sensor, though, varies from vehicle to vehicle. Still, you should be able to glean enough information from the scanner’s readout to determine if you have a sensor-related problem.

Nonetheless, you should realize that other problems may register the same diagnostic code as a faulty oxygen sensor. For example, you may have trouble with your vacuum system. As such, before replacing your O2 sensors, you likely want to inspect your vehicle’s engine for other problems.

Replacement Options

If you are unable to determine if you have a bad O2 sensor from your OBD-II scanner or a visual inspection, you likely want to take your vehicle to a reliable mechanic. You want to visit someone who has a diagnostic scan tool. This mechanic can tell you with virtual certainty whether you need to replace your O2 sensor. He or she also can likely pinpoint which oxygen sensor has failed.

When it comes to replacing a faulty O2 sensor, you have a couple options. Because sensors are plug-and-play, you may be able to do the replacement yourself. That said, sensors are not always easy to access. Even worse, sometimes the old sensors can seize in place, making them very difficult to remove.

Total Cost

The total O2 sensor replacement cost depends on whether you do your job yourself. If you do, you only need to budget for the price replacement sensors, but you will need to clear the codes with an OBD tool (Rent through Loan-A-Tool®). This price, of course, varies from vehicle to vehicle. Many sensors, nonetheless, cost between $20 and $100. Your mechanic’s labor costs also likely vary based on a few factors, including how difficult it is to access your sensors. Typically, labor costs for a sensor replacement are around $150.

Your Next Step

Keeping your vehicle’s oxygen sensors in tip-top shape is critical. After all, if you have a bad or failing O2 sensor, you could get reduced gas mileage or even fail an emissions inspection.

If you have noticed some bad O2 sensor symptoms and are ready to replace your sensors, AutoZone has everything you need to do the job right. Order sensors, tools, and other components online or visit your convenient neighborhood store for assistance. Of course, if you have questions about your O2 sensors or anything else automotive, our knowledgeable AutoZone associates are always ready to help.




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