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

Oxygen sensors record an exhaust’s O2 quantity 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 an 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 DC volts, and connect its black probe to the battery’s negative terminal and its red probe to the plug’s heater wire. The voltage should be above 12 volts.

To test the heater wires: Set the multimeter to measure 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 DC volts, connect the red probe to the blue signal wire and the black probe to the 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.

4-Wire O2 Sensors

The 4-Wire Oxygen Sensor

Oxygen sensors come with either 1, 2, 3, or 4 wires (see the 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 diagram
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, a leaking gasket, worn-out piston rings, or general bad maintenance.

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

I’ve further explained more detailed information about causes and symptoms and explained how a 4-wire oxygen sensor works.

Testing an O2 Sensor with 4 Wires

I will show how to conduct the following three tests for an oxygen sensor with 4 wires:

  • Test 1-2: Testing the input voltage and heater wires – We will test the battery’s voltage to see if the sensor is getting sufficient voltage in Steps 1-3 and the resistance in Steps 4-6.
  • Test 3: Testing the signal wires – We will test the voltage level in three steps.

Doing these tests will reveal whether you have a properly working oxygen sensor. Before starting the tests, however, have these items ready: A multimeter and a few back probes.

Test 1-2: Testing the Input Voltage and Heater Wires

A person is holding an oxygen sensor's two same-colored wires
Video | Car Hospital

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

Step 1: Locate the Oxygen Sensor

A multimeter and the oxygen sensor encircled in red
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

Disconnect the plug, which connects the oxygen sensor to the battery.

It will allow you to check the line that comes from the battery.

A multimeter to test the voltage of an oxygen sensor
Video | Nailed IT

A multimeter at the top of an open hood car with the oxygen sensor
Video | Nailed IT

Now 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 measure DC volts.
  • 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 in the oxygen sensor
Video | Nailed IT

Step 3: Interpret the Reading

A person holding and showing the multimeter's reading at 124.3v
Video | Nailed IT

We will now interpret the multimeter reading to test the O2 sensor with 4 wires in the first of the expert tests.

If the voltage is above 12V, the power supply is working fine. Switch the ignition OFF after checking the battery’s voltage.

Step 4: Set the Multimeter to Resistance Mode

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 5: Test the Heater Wires

Locate the two wires of the same color on the oxygen sensor.

As mentioned earlier, they are either white or black, i.e., of the same color, whether white or black (not mixed).

Connect the black and red probes to the two same-colored wires on the oxygen sensor.

Use the back probes if you cannot reach the plug terminals with the multimeter probes.

Step 6: Check the Reading

Finally, check the resistance reading on the multimeter. If the value is between 10 and 20 ohms, it is sufficient for the oxygen sensor’s heater to operate normally.

Test 3: Testing the Signal Wires

A person holding an 02 sensor's signal and ground wires
Video | Car Hospital

We will test the two signal wires in this second oxygen sensor test.

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

Step 1: Set the Multimeter to Measure the DC Voltage

First, prepare the multimeter and set it to measure DV volts in voltage mode.

When measuring DC volts, turn the dial to the VDC position.

Step 2: Test the Signal Wires

A person wearing an orange gloves testing the signal wires using multimeter
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 one to the other single wire, whether white or black.

Note: The wire colors might vary depending on the oxygen sensor’s model. The two same-color wires are the heater wires, which we tested earlier in Test 1, so the remaining wire is the signal ground wire. It will not have the same color as any other wire. Connect the black probe to this lone signal wire.

Step 3: Check the Reading

A person with orange gloves testing the signal wire with multimeter at 066v reading
Video | JeepSolid

Reasons for Oxygen Sensor Malfunction

A mechanic holding O2 sensor
Video | Car Hospital

There are several possible reasons for O2 sensor failure.

Oxygen sensors often get clogged due to byproducts from the fuel. As mentioned earlier, when an oxygen sensor cannot provide the correct information to the ECU, the 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.

The sensor will show wear and tear around the 60,000-mile mark, although some higher-quality sensors might last longer. The lifespan also depends on how well it is maintained besides its quality.


Oxygen sensors deal with lots of fuel byproducts.

Because of this, they can easily get contaminated, eventually leading to a completely failed O2 sensor. Lead, sulfur, and fuel additives are common byproducts of fuel. These byproducts can disrupt an oxygen sensor’s operation.

Higher Temperatures

Oxygen sensors get exposed to lots of exhaust gas, and higher exhaust temperatures can affect an oxygen sensor badly.

Frequently high temperatures will reduce an oxygen sensor’s lifespan greatly. So, you might get a bad oxygen sensor at only 15,000 miles, which would be much sooner than normal.

Bad Maintenance

Proper maintenance is essential for any vehicle.

Without proper and timely maintenance, your vehicle parts can quickly break down. The same applies to oxygen sensors. The sensor might go bad suddenly.

So, maintain your vehicle regularly to avoid such issues.

Low-Quality Fuel

Low-quality fuel, including the oxygen sensor, can affect a vehicle’s entire fuel system.

A low-quality fuel might produce many sulfur, lead, and oil ashes. These byproducts can reduce an O2 sensor’s lifespan drastically.

Leaking Gasket

A leaking gasket can produce silicate.

If silicate formation occurs, it can eventually disrupt or block an oxygen sensor’s operation. Over time, the oxygen sensor could fail.

Worn Out Piston Rings

Worn-out piston rings produce harmful phosphorous when in contact with engine oil.

The phosphorous can damage an oxygen sensor. Apart from worn-out piston rings, harmful phosphorous can also occur due to cracked cylinder blocks, broken valve guides, and a cracked combustion chamber.

Symptoms of Bad Oxygen Sensors

Now that you know the causes of a bad oxygen sensor, you can minimize the damage if you identify a bad oxygen sensor early enough.

So, here are some signs 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 due to a bad 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.

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 can discombobulate an engine’s firing time.

Also, if you’re facing 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 a 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 (lack of O2), the oxygen sensor will generate a voltage of 8000-1000mV.

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


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Video References:


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About Alex Robertson

AvatarCertifications: 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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