All About Oxygen Sensors

The oxygen sensor in your vehicle monitors the exhaust gasses emitted by the engine to check the air/fuel mixture, and sends a signal back to the on board computer to adjust accordingly. The average oxygen sensor will last 30,000 to 50,000 miles before it begins to foul and read improperly.

The ideal air to fuel mixture in most vehicles is 14.7:1, and the oxygen sensor will try to maintain this optimal ratio. When the oxygen sensor is not working properly, symptoms such as rough running, overheating, and poor fuel economy may be present. Following are some tips to locate and troubleshoot the oxygen sensor.

Where is it?

The oxygen sensor is located in the exhaust pipe, in between the exhaust manifold (engine) and the catalytic converter. Some vehicles have two, and the second one will likely be located after the catalytic converter, but for our purposes we'll just deal with the first one. They act identically.

An oxygen sensor looks kind of like a spark plug with a sensor wire coming off the end of it. If you plan on taking the oxygen sensor out for any reason, be careful not to burn yourself, because that exhaust can be hot!

The Eyeball Test

Remove the oxygen sensor by unscrewing it from the threads in the exhaust pipe. Inspect the tip of it to see if there are black or very dark carbon deposits on it, which will inhibit its ability to accurately read the air/fuel mixture. I have heard of success by taking a mild wire-brush to the tip, getting rid of any deposits that may have built up there, but it is safer to just replace the unit.

The Voltage Test

If you have a digital voltmeter, now is the time to dust it off and put it to use. With the engine idling and warm, test the connection point at the top of the oxygen sensor with the voltmeter. If you get a reading of 0.1 - 0.4 volts, the engine is running lean, meaning too much air. Check for vacuum leaks on the engine, paying special attention to the small black vacuum lines to make sure none are disconnected. A voltmeter reading of 0.6 - 0.9 means that the engine is running rich, which is too much fuel. The optimal reading at the oxygen sensor tip will be .05 volts. Before you take your vehicle to the mechanic if you get a lean or rich reading, try a new oxygen sensor - the old one may not be reporting accurately!

If you replace the oxygen sensor, be sure to use a graphite anti-seize compound on the threaded part, because they tend to cement themselves in the exhaust pretty strongly. Also be aware that the oxygen sensor an auto parts store gives you for the vehicle may differ slightly in appearance from the stock unit. This is usually not a problem, but may require some slight wiring modifications.