How To Test An Oxygen Sensor: Tips And Warnings!

In a gasoline engine, an oxygen sensor or lambda probe is used to ensure that the proper ratio of air and fuel reaches the engine. In this article we will describe how to test an oxygen sensor while it’s still in the vehicle, so read to the end.

Some car manufacturers recommend that the oxygen sensors should be replaced once every 100,000 miles, whether they are working or not. However, this can be very expensive, especially since many of the newer cars and trucks have up to 4 different O2 sensors.

When to test the O2 sensor or lambda probe?

Usually, a check engine light will provide the first indication that you might have a bad oxygen sensor. Often the engine code simply indicates that there is a problem with the O2 sensor, or it may say that there is a “malfunction of the heating circuit“.

Since lambda sensors can be very expensive and the problem might not be with the oxygen sensor itself but could be various other factors, it’s a good idea to test your O2 sensor before replacing it.

Oxygen sensors can be tested both in the car and out of the car, but many prefer to leave the oxygen sensor in the vehicle. However, by removing the oxygen sensor, you can visually inspect the sensor, which can often give you a good indication that there is a problem, and testing the sensor is a bit easier as well.

Steps to test an oxygen sensor

How to Test a Lambda Sensor
How to Test a Lambda Sensor

The steps that you must carry out to test an Oxygen sensor or a lambda probe while it is in the vehicle are the following:

  1. Begin by visually inspecting the wires leading to and from the Oxygen sensor.. Make sure the wires are well routed away from the exhaust components and also that the wires are not damaged.
  1. Then start the car and let it run until the vehicle is warm, which usually takes about five minutes. Oxygen sensors need to be at about 600 degrees Fahrenheit to provide an accurate reading. This will make the engine very hot, so be careful.
  1. Connect the rear probe to the oxygen sensor signal wire. Be very careful when using the rear probe, because the plastic connectors are often very brittle and can break easily.
  1. Connect the positive lead of the digital voltmeter to the return probe.
  1. Connect the negative lead of the digital voltmeter to a good solid ground point on the vehicle’s chassis.
  1. Turn on the voltmeter and set it to the 1 volt scale. The oxygen sensor voltage will fluctuate between 100 and 1,000 millivolts, which is 0.1 and 1.0 volts, so having a quality volt meter is important.
  1. Restart the car and check the voltmeter reading. The reading should fluctuate rapidly. If the reading stays around 0.5 volts, make sure the vehicle is fully warmed up. If the car is hot and the oxygen sensor reading does not change, then there is a problem with the lambda probe and you can stop the test.
  1. Next, create a vacuum leak to ensure the oxygen sensor responds properly to a lean air-fuel mixture.. Most vehicles have a vacuum port on top of the engine, which can be opened to create a vacuum leak. The creation of a vacuum leak should cause the sensor voltage to drop below 0.1 volts and then rise above 0.5 volts when the leak is subsequently plugged.
  1. If your car doesn’t have a vacuum port or creating a vacuum leak causes the car to stall, you can perform a propane enrichment test. This is done by adding some propane gas to the air intake, which should cause the oxygen sensor voltage to rise rapidly.
  1. As an alternative to a propane enrichment test, you can also slightly close the choke, which should cause the sensor voltage to rise rapidly.

After testing the oxygen sensor, if during steps 8 through 10 the voltage does not change or does not change as expected, then the oxygen sensor is bad and should be replaced.

How to Test an Oxygen Sensor
How to Test an Oxygen Sensor


Keep in mind the following tips:

  • Rear probes are used to check the voltage of a circuit from the rear so that the circuit is still complete.
  • If you don’t have a rear sensor, you can use a small jumper wire, but be careful not to damage the plastic connector on the oxygen sensor.
  • If you don’t have a check engine light scan tool, most auto shops can pull the code for you for free. However, you can usually only do this on cars that were made in 1996 or later.
  • If you pull the oxygen sensor out for a visual inspection, the sensor should be light brown in color. If it’s red, white, black, or light gray, it’s likely that the oxygen sensor is already bad, but you should still test it.


You should be careful about the following:

  • If the exhaust system and engine are very hot, you must be careful to avoid burns.
  • Make sure neither the rear probe nor the positive lead of the digital voltmeter come in contact with the exhaust manifold or exhaust pipe.

Items you will need

The items you will use are:

  • High impedance digital voltmeter
  • bottom probe

Leave a Comment