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Driving with a faulty O2 sensor may trigger the check engine light, cause performance problems, and eventually damage your catalytic converter. But how would you know if it is really one of your O2 sensors that is causing problems?

Testing Your Oxygen Sensor: How-To Guide

Before you start testing your O2 sensors, use a code reader or scan tool to get the diagnostic trouble code (or codes). These will help you identify the issue that’s affecting your vehicle.

Next, do some research to find out what the code/s mean (take a look at our OBD-II code directory for in-depth guides on codes related to the O2 sensor). If you get a code that points to the O2 sensor, such as a P0135 or a P0136, then there may be no need to perform an O2 sensor test.

These tests below are only general guidelines. Take note that results may vary depending on your car’s year, make, and model.

Digital Voltmeter Testing

auto mechanic using red voltmeter in focus
To test an oxygen sensor through digital voltmeter testing, you’ll need a digital multimeter (DMM).

How do I test an oxygen sensor through digital voltmeter testing? To do this test, you’re going to need a digital multimeter (DMM).

Note: This test is for traditional, zirconia O2 sensors (not wideband air/fuel ratio sensors).

  1. Set your multimeter to the millivolt (mV) DC setting or 2 DC Volts setting.
  2. Make sure your engine is switched off. Connect the red lead of the voltmeter to the O2 sensor’s signal wire using a back probe test lead.
  3. Take the black lead and connect it to a good ground.
  4. After connecting the leads, start the engine until it’s warmed up and reaches close-loop operation. This is the point where the air-fuel mixtures are adjusted based on real-time O2 sensor data.

Note: Ideally, the voltage readings from the upstream O2 sensors would be continually changing from below 300 mV to above 800 mV because of the effort to control the fuel mixture. On the other hand, the downstream (after the catalytic converter) sensor should produce a fairly steady voltage.

Here’s a video to give you an idea of how the test is done:

If your upstream O2 sensor continuously gives out a voltage that is consistently about 450 mV, this may mean that your oxygen sensor is not responding to the fuel mixture.

On the other hand, if the O2 sensor gives out a voltage that is constantly above 550 mV, that means that there is too much fuel mixture or your O2 sensor might be contaminated.

False High

Take note that a high reading could also be a result of other factors, such as:

  • Additives in the engine coolant caused by silicon poisoning
  • A loose O2 sensor ground connection
  • An EGR valve that is stuck open
  • A break contamination of the wirings and its connection
  • A spark plug that is too close to the oxygen sensors

If your vehicle’s O2 sensor gives you a voltage reading that is consistently below 350 mV, it may also indicate that something is wrong with your sensors or that the fuel mixture in the system is too lean.

False Lean

Just like a false high reading, some situations could also give out a false lean result. A defective spark plug or an exhaust leak, for example, may cause a consistently low reading.

Although a good voltmeter usually provides accurate results, this may require further analysis of other engine components. Most of the time, these symptoms could also be related to other malfunctions inside your engine. Follow these steps to rule out other issues, such as rich and lean condition:

Testing the O2 Sensor’s Response to Lean Fuel Consumption

  1. Detach the hose from the positive crankcase ventilation (PCV) valve (or another large vacuum hose).
  2. Check the voltmeter. It should read around  200 mV to 300 mV in response to the increased air in the engine. If it doesn’t, there’s something wrong with the O2 sensor.

Testing the O2 Sensor’s Response to Rich Fuel Consumption

  1. Ensure the PCV hose is connected.
  2. Detach the plastic hose connection to the air cleaner assembly.
  3. Use a rag to close off the hose connection opening to inhibit airflow to the engine.
  4. Check the voltmeter. It should read close to 800 mV in response to the reduced oxygen entering the engine. If it doesn’t, there’s likely something wrong with the O2 sensor.

Scan Tool Testing

If you have a scan tool with you, you may also use it to test the performance of your upstream O2 sensor. To perform this test, you need to run your engine at 2,000 RPM and observe your oxygen sensor voltages.

If your O2 sensor is in good condition, it should react to oxygen content and change voltages quickly.

Auto mechanic using black car diagnostic scan tool
If you have a scan tool with you, you may also use it to test the performance of your upstream O2 sensor.

Follow these steps:

  1. Connect your scan tool and start the engine.
  2. Open the engine at a fast idle (2,500 RPM) for about two minutes to allow the oxygen sensors to adjust and warm up to its operating temperature.
  3. Make sure your vehicle is in closed-loop operation by observing the activity in the scan tool.
  4. Select the “snapshot” mode in your scan tool.
  5. Hold the engine speed steady.
  6. At this point, you may start recording.
  7. Once done, review the snapshot. For each snapshot frame, place a marker beside each oxygen sensor voltage range.
  8. It’s time for some analysis. Ideally, the result should have the most snapshot for both ends (0 to 300mV and 600 to 1,000 mV). If you observe that most of the readings are in the middle, there’s a good chance that your upstream O2 sensor is not working properly.

Visual Inspection After Replacement

After replacing your oxygen sensor, it is recommended to visually inspect your old sensor. This may help you identify the cause of the problem. Sometimes, replacing your oxygen sensors with a new one will not entirely solve the problem. To keep your oxygen sensor from getting damaged again, you or your mechanic must identify the root cause of the problem.

If you would like to do this yourself, here are things you should look for in your old sensor:

  • Black sooty deposits in your old sensors are usually indicative of a rich air-fuel mixture.
  • White chalky deposits indicate silica contamination. Usually, this is caused by using the wrong type of silicone sealant when servicing the engine. It may also be caused by silica deposits in your fuel.
  • If you notice a white sandy or gritty deposit in your old sensor, this may mean an antifreeze or ethyl glycol contamination. This contamination is usually caused by a defective cylinder head or intake manifold gasket. A cracked cylinder head or engine block may also contribute to the problem. Take note that antifreeze contamination may also cause your oxygen sensor to turn green due to the antifreeze dye.
  • A defective PCV system may also cause your oxygen sensor to go bad. If you notice dark brown deposits on your old O2 sensors, they may be caused by excessive oil consumption due to a defective PCV or other mechanical engine problems.

Other Tips for Testing the O2 Sensor by Richard McCuistian, ASE-Certified Mechanic

Method 1

If you’re dealing with an O2 sensor situation and you believe the sensor might need replacing, you can watch the sensor on the scan tool data screen with the throttle at about 1200 rpm and you should see about three switches from rich to lean per second, with the voltage range being at least from 0.2 to 0.8 volts or slightly greater. While holding the throttle and watching the scan tool with the sensor switching this way, have an assistant remove a vacuum hose – the O2 sensor should drop into the lean range and the Short Fuel Trim should increase.

Have that same assistant gently and safely spray a bit of carburetor cleaner into that hissing vacuum leak and you should see the Short Fuel Trims correct in the opposite direction due to O2 voltage quickly rising.

Method 2

You can also backprobe the O2 sensor signal wire (make sure you know which one it is) and measure O2 voltage with a voltmeter set on the 2 volt scale while performing the same tests.

Realize that fuel trims and O2 activity are greatly affected by the MAF sensor and any unmetered air leaks (such as a cracked air inlet hose or even a missing oil filler cap gasket). Further, if your scan tool reads BARO pressure, make sure the pressure your scan is reading matches actual barometric pressure. If the ECM/PCM thinks the vehicle is at high altitude, it will cause a lean condition and positive fuel trims due to a skewed O2 sensor input.

You can also put your O2 sensor in a vise, check the two wires feeding the heater (both of those wires will be white, black, or brown usually), and see if the heater has about 5 ohms or so.

Next, with the sensor still in a vise (with the tip exposed) safety glasses on and leather gloves on your hands, connect a voltmeter or a scope between the other two sensor wires (gray is ground and black is voltage usually, believe it or not), and use a propane bottle torch (get help if you don’t know how to use the torch) to heat the sensor – and you’ll begin to see a voltage on your meter or scope when you get the sensor bulb to its operating temperature.

As long as you keep the propane flame at the tip of the sensor it’ll create voltage, and the instant you move the flame, the voltage will drop. Again, it should be nearly 1 volt with the flame at the sensor tip and near zero volts when the flame is moved away from the tip.

A bad O2 sensor will almost always trigger your check engine light. Sometimes the DTC will indicate a slow or sluggish sensor, which almost always means you need a replacement sensor. Also, the heater tends to burn out, which will throw heater-related codes.

Check Engine Light triggered by faulty part
A bad O2 sensor will almost always trigger your check engine light.

Zirconia O2 Sensor Basics

The ceramic zirconia bulb in a conventional O2 sensor is basically an electrolyte that creates its own voltage based on the amount of oxygen inside the bulb (from atmosphere) as compared to the oxygen outside the bulb (in the exhaust stream). The sensor is very carefully calibrated to produce near 1 volt when the exhaust stream is rich and near zero volts when the O2 sensor is lean. The kicker is that the O2 sensor must be 600°F before it will work properly.

Newer wide band air/fuel sensors need to be at about fourteen hundred degrees and operate on a different principle.

Jeep 4.0L engines from 87 to 91 used a Titania sensor that broke all the rules by reading 5 volts when the system was lean and 0 volts when it was rich – those sensors actually measure the temperature of the exhaust to determine whether the combustion gasses are rich or lean. Don’t dwell too much on those unless you own an 87-91 Cherokee or Wrangler.

Dodge and Chrysler, and later model Jeep vehicles have a peculiar algorithm whereby they feed a voltage to the O2 sensor, and if that voltage hangs at just over .5 volts (which is the target voltage for a perfect mix), the fuel trims can saw-tooth in a way that causes the engine to buck and jump.
The point is that the conventional zirconia sensor, while still very common, isn’t exactly the same on every make and model, so make sure you know what you’ve got before attempting any tests or doing any troubleshooting. Mechanics that work on vehicles every day can spot a bad O2 sensor more readily than a person who doesn’t deal with troubleshooting every day.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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