How to Test O2 Sensor with 4 Wires (2 Expert Tests)

Today I’m going to teach you how to test an 02 sensor with 4 wires. 

Oxygen sensors play a vital role in your car. These sensors record the O2 quantity of the exhaust and send it to the electronic control unit (ECU). The combustion chamber takes this information and uses it to optimize the air-fuel ratio of your vehicle. As you can imagine, having a bad oxygen sensor will affect the engine’s performance. If you learn how to test your sensors you can avoid any possible future issues that may crop up.

Unfortunately, many people don’t learn how to test their oxygen sensors and end up paying the price later on – as the saying goes one ounce of prevention is worth a pound of cure.

In general, for a 4 wire oxygen sensor, there are two tests. Here are the steps for those tests.

  • First, locate the oxygen sensor on your vehicle.
  • Disconnect the plug to expose the wire terminals of the O2 sensor.
  • Check the battery voltage on the plug that comes from the battery.
  • Set the multimeter to resistance mode.
  • Test the two white/black color wires for ohms. The value should be between 10 – 20 ohms.
  • Set the multimeter to voltage mode.
  • Test the other two wires for voltage. The value should be between 0.1 – 0.9 V.

We’ll go into more detail below.

A Complete Guide on How to Test an O2 Sensor with 4 Wires

In this guide, I’m going to talk about two different tests. Completing both these tests will ensure a properly working oxygen sensor. Before starting the guide, here are a few items you’ll need for this.

Things You’ll Need

  • Multimeter
  • Few back probes

Test 1 – Testing the Heater Wires

In this test, we are going to test the two heater wires. Here’s how you can do that.

Step 1 – Locate the Oxygen Sensor

locating the oxygen sensor
Video | Nailed IT

First and foremost, you need to find the location of the O2 sensor. Usually, the O2 sensor is located in the exhaust gas flow. It should be close to the engine. So, find the location of the sensor correctly.

Step 2 – Test the Battery Voltage

testing the battery voltage
Video | Nailed IT

Then, disconnect the plug. This plug connects the battery and the oxygen sensor. So, when you disconnect the plug, you’ll be able to check the line that comes from the battery.

disconnecting the plug between battery and oxygen sensor
Video | Nailed IT

  1. To check the battery voltage, turn the ignition key to the ON position (do not start the vehicle).
  2. Set the multimeter to voltage mode.
  3. Connect the black probe to the negative battery terminal.
  4. Connect the red probe to the plug’s black wire (heater wire). You may have to use a back probe for this.

connecting multimeter probes
Video | Nailed IT

multimeter a 124.3v reading
Video | Nailed IT

If the voltage is above 12V, the power supply is working smoothly.

Keep in mind: After checking the battery voltage, switch OFF the vehicle.

Step 3 – Set the Multimeter to Resistance Mode

Next, take the multimeter and set it to resistance mode. Turn the dial to the Ω mark. Also, connect the blackjack to the COM port and the red jack to the V/Ω port.

Step 4 – Test the Heater Wires

Now locate the two wires that have the same color on the oxygen sensor. As mentioned earlier, they should be white or black.

Then, connect the black and red probes to those two wires. If you are unable to reach the plug terminals with the multimeter probes, use the back probes.

Step 5 – Check the Value

Finally, check the reading on the multimeter. If the value is between 10 – 20 ohms, the oxygen sensor’s heater is working.

Test 2 – Testing the Signal Wires

Here, we’ll test the two signal wires. This testing process is quite similar to test one.

Step 1 – Set the Multimeter to Voltage Mode

First, take the multimeter and set it to voltage mode. Here, we are measuring DC volts. So, turn the dial to the VDC position.

Step 2 – Test the Signal Wires

testing the signal wires
Video | JeepSolid

Next, turn on the engine. Then, connect the multimeter probe to the two wires. Connect the red probe to the blue wire. And the black probe to the white wire.

Tip: The wire colors might vary depending on the model of the oxygen sensor.

Step 3 – Check the Reading

multimeter at 066v reading
Video | JeepSolid

The reading should be between 0.1 and 0.9 volts. If the voltage reading is inside that range, the sensor wires are working properly.v

Reasons for Oxygen Sensor Malfunction

mechanic holding O2 sensor
Video | Car Hospital

There are plenty of reasons for O2 sensor failures. Most of the time, the sensor gets clogged due to byproducts that come from the fuel. As mentioned earlier, when the oxygen sensor is unable to provide the correct information to ECU, your engine will start to show bad signs. So, here are some reasons that can lead to bad oxygen sensors.


Most oxygen sensors have a lifespan of 60000 to 90000 miles. So, the sensor will show wear and tear at the 60000-mile mark. Some sensors might last more. Usage time depends on the quality of the sensor and the maintenance level.


These oxygen sensors deal with lots of fuel byproducts. Because of this, they will get contaminated. It might lead to a completely failed O2 sensor. Lead, sulfur, and fuel additives are common byproducts of fuel. These byproducts will disturb the process of the oxygen sensor.

Higher Temperatures

Oxygen sensors get exposed to lots of exhaust gas, and higher exhaust temperatures can affect the oxygen sensor badly. It will reduce the life span of the sensor greatly. So, you might go bad oxygen sensor at 15000 miles.

Bad Maintenance

Proper maintenance is a must-follow step for any vehicle. Without proper and timely maintenance, your vehicle parts will suddenly break down. The same theory applies to the oxygen sensor. The sensor might get bad quickly. So, maintain your vehicle regularly to avoid such issues.

Low-Quality Fuel

Use of the low-quality fuel will affect the entire fuel system, including the oxygen sensor. A low-quality fuel might produce a large amount of sulfur, lead, and oil ashes. These byproducts reduce the life span of the O2 sensor drastically.

Leaking Gasket

If you are dealing with a leaking gasket, it might produce silicate. Eventually, silicate will block the process of the oxygen sensor. With time, the oxygen sensor might fail completely.

Worn Out Piston Rings

Worn-out piston rings produce harmful phosphorous with the engine oil. This phosphorous can damage the oxygen sensor. Apart from the worn-out piston rings, here are a few ways that harmful phosphorous can occur. (1)

  • Cracked cylinder blocks
  • Broken valve guides
  • Cracked combustion chamber

Symptoms of Bad Oxygen Sensors

You now know the causes of a bad oxygen sensor. However, if you can identify the bad sensor early, you can minimize the damage. So, here are some signs that you should look out for.

Check Engine Light

If the check engine light is ON in your vehicle, it is an indication of the emission problem. It might be the oxygen sensor.

Poor Fuel Economy

A bad oxygen sensor can cause fuel economy issues. It will lower the fuel economy to a great extent. So, whenever you detect poor fuel economy, remember to check the O2 sensors. (2)

Engine Misfiring and Sluggish Performance

Engine or cylinder misfiring is one of the direct indications of a bad oxygen sensor. These sensors help the engine to control its timing. So, a bad oxygen sensor might discombobulate the firing time of the engine.

Also, if you are dealing with unusual engine performance issues, it might cause by the failed O2 sensor.

Wiring Diagram for Oxygen Sensor

02 sensor wiring
Video | ADPTraining

Before going into the how-to part, understanding the oxygen sensor wires of an O2 sensor will help you immensely. Oxygen sensors come with 1, 2, 3, or 4 wires. in this article, we focus on the 4-wire oxygen sensor.

oxygen sensor's two same-colored wires
Video | Car Hospital

Most oxygen sensors have two same-colored wires. The color of these two wires can be white or black (mostly white). These two wires are the heater wires.

02 sensor's signal and ground wires
Video | Car Hospital

The color of the remaining two wires might vary according to the manufacturer. However, from those two wires, the blue one is the signal wire, and the other is the ground wire.

Keep in mind: Always remember the above guidelines while using a 4-wire oxygen sensor. Correctly identifying the oxygen sensor wires is a must.

How Does a 4 Wire Oxygen Sensor Work?

The oxygen sensor is located throughout your vehicle’s exhaust system. Some cars might have two O2 sensors.

Usually, oxygen sensors can categorize into two types; wide and narrow bands. Either way, the sensing element is located inside the sensor. The sensing element is covered with steel housing. When the O2 molecules run through the exhaust system, they reach this sensing element. If the ratio of air-fuel is rich (not enough O2), the sensor will generate an 8000-1000mV.

Tip: The oxygen sensor will start to operate at 650°F.

Wrapping Up

Oxygen sensors are a crucial part of modern-day vehicles. One sensor might not be enough for some vehicles. Anyway, use the above-discussed methods to test the oxygen sensors regularly. If the sensor is bad, replace it with a brand new one. However, without proper maintenance, the new sensor might face the same fate as the old one. So, maintain the vehicle according to the service intervals.

Take a look at some of our related articles below.

(1) harmful phosphorus –
(2) fuel economy –

Video References


Nailed IT

Jonathan Jiang


Car Hospital

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About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gCertifications: B.E.E.
Education: University Of Denver - Electric Engineering
Lives In: Denver Colorado

Electrical engineering is my passion, and I’ve been in the industry for over 20 years. This gives me a unique ability to give you expert home improvement and DIY recommendations. I’m not only an electrician, but I also like machinery and anything to do with carpentry. One of my career paths started as a general handyman, so I also have a lot of experience with home improvement I love to share.

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