How to Test an O2 Sensor with 4 Wires (3 Expert Tests)

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

To test a 4-wire oxygen sensor’s input voltage: Disconnect its plug, turn the ignition on (without starting the vehicle), set the multimeter to read DC voltage, and connect its black probe to the battery’s negative terminal and its black probe to the plug’s heater wire.

→ The voltage should be above 12 volts.

To test the heater wires: Set the multimeter to read resistance and connect the two probes to both wires.

→ The resistance should be between 10 and 20 ohms.

To test the signal wires: Set the multimeter to read DC voltage, connect the red probe to the blue signal wire, and the black probe to the remaining signal ground wire.

→ The voltage level should be between 0.1 and 0.9 volts.

I’ll go into more detail below on how to test an O2 sensor with 4 wires.

Testing an O2 Sensor with 4 Wires

The 4-wire Oxygen Sensor

Oxygen sensors come with either 1, 2, 3, or 4 wires (see picture below).

Here, we focus on the 4-wire oxygen sensor. It typically has two same-colored heater wires, usually in either white or black, and two signal wires, usually blue for the signal and a different color for the signal ground.

02 sensor wiring
Video | ADPTraining

Causes and Symptoms of Malfunctioning

An oxygen sensor can malfunction due to various reasons.

They include mileage, contamination, high temperature, low-quality fuel, leaking gasket, worn-out piston rings, or general bad maintenance.

You can tell if the oxygen sensor is bad if you notice a symptom such as a check engine light or experience poor fuel economy, sluggish performance, or engine misfiring.


I’ve given more detailed information about causes and symptoms further on. I’ve also explained how a 4-wire oxygen sensor works. But first, I will show you how to conduct the following two tests:

  • Test 1: Testing the heater wires – We will test the battery’s voltage to see if the sensor is getting sufficient voltage in Steps 1-2, and the resistance in Steps 3-5.
  • Test 2: Testing the signal wires – We will test the voltage level in three steps.

Completing both these tests will ensure a properly working oxygen sensor. Before starting, have these items ready: A multimeter, and a few back probes.

Test 1: Testing the Heater Wires

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

In this first test, we will test the two heater wires. Here’s how to do that:

Step 1: Locate the Oxygen Sensor

locating the oxygen sensor
Video | Nailed IT

First and foremost, you need to locate the O2 sensor. It’s usually located close to the engine in the exhaust gas flow.

Step 2: Test the Battery Voltage

testing the battery voltage
Video | Nailed IT

Disconnect the plug, which connects the oxygen sensor to the battery. This will allow you to check the line that comes from the battery.

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

Then test the battery’s voltage as follows:

  • Turn the ignition key to the ON position (but don’t start the vehicle).
  • Set the multimeter to voltage mode.
  • Connect the black probe to the battery’s negative terminal.
  • Connect the red probe to the plug’s heater wire (white or black). 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 fine. Switch the ignition OFF after checking the battery’s voltage.

Step 3: Set the Multimeter to Resistance Mode

Next, set the multimeter to resistance mode. Turn the dial to the Ω mark. Then, connect the blackjack to the COM port and the red jack to the Ω port.

Step 4: Test the Heater Wires

Locate the two wires that have the same color on the oxygen sensor. As mentioned earlier, they are either both white or black, i.e., in the same color whether white or black.

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

Step 5: Check the Value

Finally, check the resistance reading on the multimeter. If the value is between 10 and 20 ohms, the oxygen sensor’s heater is operating normally.

Test 2: Testing the Signal Wires

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

Now, we’ll test the two signal wires.

The process is similar to Test One, except we will test the signal wires.

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

Now, turn on the engine.

Then, connect the multimeter probes to the two wires. Connect the red probe to the blue wire and the black probe to the other single wire, whether it is white or black.

Note: The wire colors might vary depending on the model of the oxygen sensor. The two same-color wires will be heater wires, so the remaining wire will be the signal ground wire (which will not have the same color as any other wire) to which you connect the black probe.

Step 3: Check the Reading

multimeter at 066v reading
Video | JeepSolid

The reading should be between 0.1 and 0.9 volts. The sensor’s signal wires are working properly if the voltage reading is inside that range.

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 cannot provide the correct information to the ECU, your engine will show the check engine light. So, here are some reasons that can lead to a bad oxygen sensor:


Most oxygen sensors have a lifespan of 60,000 to 90,000 miles. So, the sensor will show wear and tear at the 60,000-mile mark. Some sensors might last longer. Usage time depends on the quality of the sensor and the maintenance level.


The oxygen sensors deal with lots of fuel byproducts. Because of this, they will get contaminated, which might lead to a completely failed O2 sensor. Lead, sulfur, and fuel additives are common byproducts of fuel. These byproducts can disrupt the oxygen sensor’s operation.

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 get a bad oxygen sensor at only 15,000 miles.

Bad Maintenance

Proper maintenance is essential for any vehicle. Without proper and timely maintenance, your vehicle parts can suddenly break down. The same applies to the oxygen sensor. The sensor might get bad quickly. So, maintain your vehicle regularly to avoid such issues.

Low-Quality Fuel

Low-quality fuel can affect the entire fuel system, including the oxygen sensor. A low-quality fuel might produce many sulfur, lead, and oil ashes. These byproducts could reduce the O2 sensor’s lifespan drastically.

Leaking Gasket

If you are dealing with a leaking gasket, it might produce silicate. Eventually, silicate will block the oxygen sensor’s operations. 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, harmful phosphorous can occur due to cracked cylinder blocks, broken valve guides, and a cracked combustion chamber.

Symptoms of Bad Oxygen Sensors

You now know the causes of a bad oxygen sensor.

However, if you can identify a 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 indicates an 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 a direct indication of a bad oxygen sensor. These sensors help the engine to control its timing. So, a bad oxygen sensor might discombobulate the engine’s firing time.

Also, if you’re dealing with unusual engine performance issues, it might be due to a failed O2 sensor.

How a 4-Wire Oxygen Sensor Works

The oxygen sensor is located along your vehicle’s exhaust system.

Some cars have up to four O2 sensors. Regardless, they can be categorized into wide and narrow bands. Either way, the sensing element is inside the sensor and covered with steel housing. When the O2 molecules run through the exhaust system, they reach this sensing element. If the air-fuel ratio is rich (not enough O2), the sensor will generate a voltage of 8000-1000mV.

Note: Oxygen sensors only operate effectively at a temperature above 600°F (~315°C).


(1) harmful phosphorus –

(2) fuel economy –

Video References


Car Hospital

Jeep Solid

Jonathan Jiang

Nailed IT

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About Alex Robertson

c3c9d43f1f0d14c4b73cb686f2c81c4e?s=90&d=mm&r=gCertifications: B.M.E.
Education: University Of Denver - Mechanical Engineering
Lives In: Denver Colorado

Hi, I’m Alex! I’m a co-founder, content strategist, and writer and a close friend of our co-owner, Sam Orlovsky. I received my Bachelor of Mechanical Engineering (B.M.E.) degree from Denver, where we studied together. My passion for technical and creative writing has led me to help Sam with this project.

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