2001 Subaru Outback - Oxygen Sensor

What Does an O2 Sensor Do?

Oxygen sensors, often also called O2 sensors, lambda sensors, or air-fuel ratio sensors, measure the oxygen concentration in your exhaust, communicating this information to the engine control module (ECM) to adjust the fuel mixture. This helps ensure that your vehicle is running as efficiently as possible, and minimizes emissions of unburned fuel that could damage the catalytic converter. Most vehicles have at least two O2 sensors: an upstream sensor located ahead of the converter, and a downstream sensor found behind the converter. In vehicles with dual exhaust or V-style engines, it's also possible for there to be two upstream and two downstream sensors as well. Fuel ratio information from the upstream and downstream oxygen sensors is interpreted by the computer to determine what adjustments are necessary for peak performance and efficiency.

Bad O2 Sensor Symptoms

The most common sign of a failing or damaged oxygen sensor is the Check Engine light appearing on the dash. This occurs because the engine computer has detected an abnormally high or low concentration of unburned hydrocarbon or fuel in the exhaust either ahead, behind, or on both sides of the catalytic converter. Your Check Engine light may blink or stay on constantly, but in either case, should be addressed to determine the source of the problem. If you believe your O2 sensors may be causing your Check Engine light, visit your local O'Reilly Auto Parts for free Check Engine light testing.

Though the Check Engine light will likely activate in any case, there are other symptoms that may accompany this warning if your oxygen sensors are failing. These include:

• Rough or erratic idle
• Misfiring or stalling
• Reduction in performance or power
• Significant decrease in fuel economy
• Increased emissions or a failed emissions test

If you notice these issues, you can be certain that something is wrong with your engine, which may include a damaged or worn O2 sensor, or a variety of other issues. If your Check Engine light test is inconclusive, further diagnosis or a professional service center may be necessary to make your repair.