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How To Test O2 Sensor In Your Car Or Truck?

An O2 sensor is located on the exhaust. It measures the percentage of oxygen present. It is a major part of the emissions system and crucial for your car’s efficient working. Are you having trouble with your car’s emission system? Do you feel your car’s o2 sensor requires a check? If yes, then this article is going to turn out very helpful as it explains how to test the o2 sensor.

The fuel/air mixture balance depends on the amount of oxygen present. Less oxygen means more fuel consumption. More oxygen means lean fuel consumption. This data is sent to the Engine Control Unit (ECU). A failed oxygen sensor would not be able to send data to the ECU. Therefore, an unbalanced fuel/air mixture is inevitable.

An unbalanced fuel/air mixture means increased pollution will be caused by the car. This will harm the environment by polluting it with dangerous and toxic emissions. The o2 sensor is very important for the well-being of the catalytic converter and the car. Henceforth, it needs to be tested and changed if required.

Signs Of A Bad O2 Sensor:

There are many signs of a bad o2 sensor and they need to be looked out for. Some of the most common signs are-

1. Check Engine Light:

When the fuel/air mixture is not balanced properly, the car’s engine gets inefficient. This causes the check engine light to blink. If you’re facing this there’s a great chance the o2 sensor might be going bad.

2. Odour From Exhaust:

When the o2 sensor is not working properly there will be toxic emissions through the exhaust and you might experience odour from the exhaust. This will be very similar to the smell of rotten eggs.

3. Delayed Start:

The car will take time to start its engine as the unbalanced fuel/air mixture will mess it up. This is another sign of a bad o2 sensor.

4. Rattling Engine:

Another consequence of a bad fuel/air mixture in the car and the engine rattling during startup.

5. Mileage:

The car’s mileage will suddenly drop down if the o2 sensor has gone bad. This can be explained by the unbalanced fuel/air mixture.

There could be other reasons for the above signs but if you’re facing more than one issue listed above, there’s a high chance the o2 sensor is going bad.

How To Test O2 Sensor?

A loose vacuum or a loose wire connecting the o2 sensor could be the reason for the high or low readings. There are around five o2 sensors in your exhaust system and it will be hard to identify which one you have to test. The computer DTC will tell you which o2 sensor you have to test. Use the manual and the DTC to locate the o2 sensor.

  • You will require a 10 megaohm impedance voltmeter as it will provide specific measurements. Set it to millivolt DC Scale.
  • Start the engine and let it reach the required temperature for operation.
  • Turn the engine off after reaching the temperature. Connect the red probe to the sensor’s signal wire. And connect the black probe to good ground. Be careful while doing this as the temperature will be high.
  • Now start the car again and check the voltmeter readings. A normally working o2 sensor should have a reading between 100mv-900mv. If this is the case with you, then you may stop testing. Continue with the test if the readings are different.
  • Disconnect the hose from the PVC valve which is on the valve cover. If the voltmeter reading is 200mv then the o2 sensor is working properly. Move to the next step if the reading is different.
  • Reconnect the hose to the PVC valve. Disconnect the plastic hose connection to the air cleaner assembly. Now cover the hose connection using a piece of cloth.
  • The voltmeter reading should be close to 800mv. If it is different the o2 sensor is not working properly.
  • Connect the hose to the cleaner

If the readings were correct during the other tests, there could be another reason for this: a vacuum leak or a loose wire. The o2 sensor needs a replacement if it is not functioning properly.

How Much Would It Cost To Replace An O2 Sensor?

An o2 sensor is going to cost from $20 to $100 depending on your car model. The included labour cost will get it up to anywhere between $200 to $500. If you think this is too much, a catalytic converter can cost more than double this amount. Being ignorant can degrade the catalytic converter and can cost you a lot.

Frequently Asked Questions:

So, here are some of the frequently asked questions about how can you tell which O2 sensor is bad, how many ohms should an O2 sensor have etc?-

How Can You Tell Which O2 Sensor Is Bad?

You can tell which O2 sensor is bad –

  1. First, you should Make sure that the engine exhaust system is not hot.
  2. After that Switch the clamp-meter on.
  3. Put the clamp around the oxygen sensor heater power wires
  4. Then, at last, Observe the reading, which may be 0.25A and 1.5A

How Many Ohms Should A O2 Sensor Have?

An O2 sensor has 10-25 ohms, If it has low resistance, it is probably the sensor.

How Do I Test An O2 Sensor With OBD2?

You can test an O2 sensor with OBD2 –

  1. Insert the OBD2 scan tool connector into the diagnostic link connector.
  2. Then Turn on your vehicle’s engine.
  3. After the scanner boots, go to the menu and select ‘Codes’.

Can You Clean An O2 Sensor?

Some people choose to remove it and use a wire brush or an aerosol cleaner to remove deposits.

What Should An O2 Sensor Read With Engine Off?

With KOEO  , look for signs of corrosion or water intrusion. Repair it if it is necessary. The voltage reading should be about 0.5 volts.

What Causes O2 Sensor Slow Response?

The slow response in the O2 sensor may be caused due to a sensor that is just getting old and over time may get contaminated with carbon, etc that deteriorate the sensor over time.


The o2 sensor is a very crucial part of the car’s emission system. It is very important and it regulates the gases emitted from the exhaust. If the o2 sensor has gone bad the car will pollute the environment. Delaying the replacement of the o2 sensor can be fatal for the catalytic converter. And, it is not going to be sweet. A catalytic converter can cost easily around $1000, whereas an o2 sensor will be comparatively cheaper. Going to a repair shop should be the priority if you’re facing the symptoms.