Oxygen Sensor Test

If your engine light came on, and you took it to the shop to read the codes, you probably got an oxygen sensor code. Now you need to know how to test the sensor, because it is too expensive to just replace and hope for the best, right? Once you are done reading this article, you will know how to test oxygen sensors straight from a professional, and not somebody who just copied another person's work.

The first step in testing an oxygen sensor looking at it and giving a thorough visual inspection. You should check the connectors, wires, sensor body, and insulator for cracks or other damage. A complete visual inspection even calls for removing the sensor and looking at the tip of it. When you do this, you are looking for a white powdery looking substance, which would be evidence of silicone contamination. If everything looks good, and you cannot see any obvious problems, the next step would be to look at the fuses. There is often a fuse for the oxygen sensor heater element, and if it is blown, it can also mean that there is a bad oxygen sensor because of the short circuit that blown the fuse. If the fuse is also good, then it is time to get out the voltmeter, and test the sensor.

Though it is possible to test an oxygen sensor with a blow torch and an ohmmeter, the testing process can damage the sensor making a good sensor completely worthless. Then you change a part, just to find that you still have the same problem. So the best way to test an oxygen sensor is while it is still hooked to a running engine. If you take your car to a shop for diagnosis, the scan tool they use can run through various tests to see how well the senor responds to changes, but most people do not have the money to take their car to a shop for something that can otherwise be done in a few minutes out in the driveway.

Hooking up the volt meter

To test the oxygen sensors, you need to find the low voltage output from the sensor. That is the signal that it sends back to the computer. It is usually going to have low voltage labeled on the wiring diagram for your car, or it will be the wire that comes off the symbol with the zigzagging line, in the wiring diagram. You need to set your meter to low voltage, like 2 volts DC (most meters do not have a 2 volt setting as it is 10 VDC, that is how I know the person with the other article copied this work to cheat in a contest), and then put the other probe on the engine block. Be careful not to unplug the sensor, but stick your probe into the connector. A paper clip works handy for this part. Now, you are ready to start the engine so you can monitor the voltage that the oxygen sensor is putting out. The next step is to see how it reacts to different situations.

Normal operation test

Everything is hooked up and good to go, now warm the engine to the normal operating temperature, and look at the reading on the meter. While you are watching the meter, have someone raise the engine speed to about 2500 RPM, or the normal speed that you drive it at for about 20 seconds. You are looking for the meter to fluctuate about half of a volt. SO an acceptable range for most vehicles would be 0.2 – 0.8 volts. The exact numbers are not what are important, it is the fact that the reading fluctuates about 7-8 times in each 10 seconds of the engine being at normal cruising speed. If it does not fluctuate then the sensor is bad. If it is at an extreme voltage then you could also have other problems in the engine like a vacuum leak, or cylinder misfire. All of that brings us to the next part of the test.

Rich system test

The next part of the test is to make the engine run rich, and see if the oxygen sensor tries to correct it with a change in voltage. When you make the system rich, the voltage on the sensor should increase, so you can try spraying extra gas in the engine, or creating a misfire to cause extra fuel to be in the exhaust system but there is actually a propane kit that is used for this in the shops. Anyways, when you make the system rich, the voltage should rise to about 0.8 volts and then return to normal when you correct the rich condition. Next we check for how it reacts to lean conditions.

Lean system test

To check the oxygen sensor’s reaction to a lean condition is much easier than checking for the rich voltages. To check for lean condition voltages, we simply create a vacuum leak in the engine. Something like disconnecting the vacuum brake booster, or the PCV valve, for a few seconds will not hurt the engine and can help you pinpoint a very elusive oxygen sensor failure. When you create the vacuum leak the voltage signal coming out of the sensor should drop to about 0.3 volts within a couple of seconds.

What’s next

If all of this tested well then you will want to look into the heating element in your oxygen sensor. To test this, you simply put the meter on ohms and measure resistance between the heater element terminals according to the wiring diagram for your vehicle. One of them comes from a fuse, and the other goes to ground. When you check it while the sensor is cold, then you will have an infinite reading. If you check it while the engine is hot then you will have a reading somewhere in the 10 – 15 ohm reading. One thing you need to know when testing this is to never check ohms on a system that has power going to it. So you will want to disconnect the sensor, and check the wires going into the sensor itself for this test.

Things to Remember

If your engine has 2 oxygen sensors, then you will also want to perform this series of tests on the other sensor. If the sensor does not fluctuate then it is bad. You will want to see the factory repair manual for your vehicle to get correct voltages and resistance for your particular vehicle, as the voltages here are based on an educated guess that comes from many years of experience. With the right amount of patience, you should be able to get through a successful oxygen sensor test by following the simple instructions that are given to you.Good luck, and hopefully now you can fix it right the first time.