How to Test an o2 Sensor with a Multimeter


The O2 sensor is an ​abreviation for Oxygen Sensor. It is an important part of our cars’ emission system. The role of the emission system is to reduce the number of harmful gasses released into the environment.

The vehicles manufactured before the 1980s have their o2 sensor fitted in their exhaust system. The core reason for having an o2 sensor in the exhaust system was to measure the amount of unburned oxygen as the gases exit the engine system.

In case the fuel mixture has excess or too little oxygen, the engine burns lean. This data is beneficial to the Engine Control Unit(ECU) system to control the fuel-oxygen ratio. In case the car is diagnosed with a faulty o2 sensor, it does not run effectively.

​Getting to Know the Component

The O2 sensor is a small, simple gadget with a tip sensor that is inserted into the exhaust pipe. The sensor is, therefore, designed to measure the ratio of oxygen in the exhaust emissions. The oxygen ratio recorded by the sensor is then sent in real-time to the ECU, which then controls the ratio of fuel and oxygen needed.

How to Use a Digital Multimeter

In the case of a faulty 02 sensors, the ECU is not able to read and adjust the oxygen-fuel levels correctly. The ECU system is designed to regulate the fuel amount entering the engine in relation to the level of oxygen measured by the o2 sensor. 02 sensor failure leads to inappropriate levels of fuel and oxygen released to the system by the ECU and as a result, increases the pollutants released by the exhaust.

This has a ripple effect not only destroying the environment but also damaging the entire car engine over time.

How to Know if the 02 Sensor is Bad

You might be wondering how you can detect a faulty oxygen sensor, let’s discuss the observable signs related to o2 failure. Though not easy to get to the o2 due to its location physically, there exist some caution signs to alert you in case of a problem. The obvious symptoms include:

Examples of Symptoms

  • Bad odor from the exhaust pipe.
  • A decrease in gas mileage.
  • The check engine light automatically goes on.
  • The car engine makes a rough idling.
  • The starter gets hard while starting the car.

The check engine light going on, with a combination of any other sign mentioned above, may be an indication of a faulty o2 sensor. To ascertain the problem, you should read the diagnostic trouble code stored in the Engine Control Unit. In case the diagnosed trouble code indicates that the o2 is failing, consider conducting further examinations.

Therefore, how will you conclude that the oxygen sensor needs a replacement? This will eventually be determined by how well you will be able to perform the O2 diagnostics. You need to use a multimeter to provide you with the actual measurements. The measured results will then need further analysis of other engine components.

There could be a possibility of a loose connection making the o2 sensor to detect high oxygen levels. There could also be a chance of a loose connection to the oxygen sensor, making it inappropriate to detect the emissions through the exhaust.

You have no other available option other than getting yourself dirty as you explore deep to the specific location of the sensor inside the exhaust pipe. Let us now explore the detailed steps to undertake while diagnosing the oxygen sensor problems.

Testing The 02 Sensor

​Follow the Steps:

  1. Locate the particular oxygen sensor that needs an o2 test. This relies on the year that your car was manufactured. Modern cars may have one to five oxygen sensors, all fitted along with the exhaust system. The experts have made it easier to identify the faulty O2 through the Engine Control Unit. The car manufacturer’s manual will help you find the sensor. The manual will also help the car user to locate the signal wire from the many wires attached to the sensor.
  2. The technician will also require to use a multimeter for testing the oxygen sensor. Set the multimeter to test the circuit using the best scale available on your device.
  3. Ignite the car and leave it to warm up until it reaches operating temperatures. Give the car approximately twenty minutes to obtain the optimal temperatures.
  4. Switch off the engine after obtaining the desired temperatures.
  5. Connect the red cable to the oxygen sensor’s signal wire and the black cable to a suitable ground. (CAUTION! connect the cables carefully as the engine system and the exhaust pipe is dangerously hot)
  6. Perform the actual test. Switch on the car again and observe the multimeter’s readings. The oxygen sensor’s readings should range somewhere between 100Mv – 900Mv. This shows that the sensor is operating normal, hence you can quit the test. If the readings do not lie within this range, then the sensor has a problem.
  7. Test the oxygen sensor to observe how it responds to the consumption of fuel scenarios.
  8. Disconnect the hose from PVC (positive crankcase ventilation) valve at the valve cover. This step will allow entry of more air into the engine such that the multimeter reads about 200Mv. In case the multimeter fails to detect appropriately, the oxygen sensor is faulty.
  9. Connect the PVC hose back.
  10. Test the oxygen sensor’s response against a high fuel consumption scenario. Disconnect the plastic hose network to the air cleaner assembly using a piece of cloth to lower the amount of air getting to the engine system.
  11. Read the multimeter. The readings are supposed to be near 800Mv due to the reduction of oxygen supply to the engine. In case the multimeter responds; differently, the o2 is faulty and needs replacement.
  12. Connect back the hose to the air cleaner.
  13. In case the oxygen sensor perfectly well to the rich and lean fuel tests, there could be a possibility of another component causing the problems. The possible defect could be in the ignition system or vacuum leak.

To Sum Up

After performing the multiple testing, you should determine if the problem is with the sensor or another component. If the o2 has the problem, you can quickly fix it out by yourself. If you are probably not sure about it, take the car to a professional mechanic. Always keep it in mind that diagnosing and fixing the problem sooner than later will save you from developing more serious issues.

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