How To Test O2 Sensor With Obd2

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O2 sensors are a fundamental part of your car’s exhaust system that plays a crucial role in emissions, fuel efficiency, and engine performance in general.

However, O2 sensors can fail or malfunction, leading to inefficiency, decreasing engine performance, or even wrecking.  The first and foremost sign of failing O2 sensors are when your “check engine” light is on, and that’s also when you need it.

While you can check the O2 sensor manually, the OBD2 scanner makes it easier and more practical. Learn How do you test an O2 sensor using an OBD2 scanner and everything you need to know about O2 sensors below.

Normal O2 sensor readings

Oxygen Sensors Location: Oxygen sensors are typically located on your car’s exhaust manifold(upstream) or near the catalytic converter(downstream), where the gas flow is exhausted.  The exhaust system may host multiple oxygen sensors that send reading to the onboard computer.

The sensor readings on the voltmeter should be firstly consistent at 0.1-0.2 volts as it’s connected. It should begin fluctuating from 0.1 to 0.9 volts after 2-3 minutes of reading.

It would be abnormal if the sensor began fluctuating after four minutes, and that could be a sign of a malfunctioning O2 sensor. You can do another round of tests, but you better replace the O2 sensor unit if it has the same results.

Why do O2 sensors fail?

O2 sensors can fail for several reasons. The leading cause is when the engine coolant or the combustion chamber are leaking, allowing contaminants like fuel additives, sulfur, or lead to enter the manifold and to crust on the sensor.

Another reason is where the O2 sensor unit has reached its lifespan or has experienced normal wear, so it can no longer provide correct reading to the car’s computer or simply stop working.

At this point, a failing O2 sensor also informs you of other problems your car might have.

How do I know if my O2 sensor is bad?

  • Checking your O2 sensors could be tricky due to their unreachable placement, but you’d undoubtedly notice signs when your O2 sensor is bad. These include:
  • Engine light is on
  • Gas mileage decrease significantly
  • The car’s engine sounds rough when it idles
  • You hardly start the engine at a sudden
  • The exhaust secret bad smell you’ve not recognized before

When you notice these signs, you can do an O2 sensor test with the help of an OBD2 scanner.

How to Test a Bad O2 Sensor with an OBD2 Scanner

First of all select an OBD2 scanner. See Our Top Collection

OBD2 scanner with abs & srs, scanner With live data (These scanners have o2 sensor facilities)

O2 Sensor with an OBD2 Scanner

  1. An OBD2 scanner offers a hassle-free solution to test a malfunctioning O2 sensor when you’re confident with manual tests. These are how to test a bad O2 sensor with an OBD2 scanner:
  2. Locate the 16-pin diagnostic link connector(DLC) or the OBD2 port on your vehicle and connect the Autel scanner properly.  Older cars may have a 20-pin socket, but it also means that you need an adapter. Ensure your vehicle compatibility before purchasing an OBD2 scan tool.
  3. Turn the ignition on but don’t start the engine. Allow the scanner to communicate with your car’s onboard system. Once the scan tool’s display shows nothing, it might not be properly connected and need reconnecting.
  4. Some OBD2  scanners come with Vehicle’s Identification Number(VIN) requirements. As it’s connected, the scanner may prompt you to input the car model and makes.  Once the scan tool is booting, you can navigate to the Code menu. 
  5. When it comes to O2 sensors, notice the voltage fluctuations as the scanner is reading them. These would allow the scanner to identify the actual performance of the sensors with one of two categories of codes, including rich running engine and lean running engine.
  6. If the Autel scanners show you a lean running engine, then you can do a propane test like previously explained. You can perform the vacuum leak test if the scanner informs you of the rich running engine codes.  Once these tests don’t fluctuate the voltage reading, you’re likely having problems with your O2 scanners. If the unit is relatively new, it may need a further diagnosis, but you may consider a replacement if the sensor is already old.
  7. Learn how to interpret the code from valid references. Some OBD2 scanners may even redirect you to their library or provide on-display interpretations.  It’s fundamental to interpret the code for taking further steps correctly.

How do you check an O2 Sensor?

Prepare some tools for the O2 sensor checking, including the back probe, quality voltmeter, propane gas, and safety equipment. These are how you can check an O2 sensor manually:  

  1. Inspect your O2 sensor wiring whether or not it’s at a prime condition and the correct position.
  2. Turn on your car for around 3-5 minutes until it’s warm enough. The idea is to reach the optimum temperature where O2 sensors can read accurately.
  3. Grab your back probe and connect it to the sensor wiring carefully and ensure they have good contact.
  4. From your digital voltmeter, connect the positive lead to the black probe while the negative lead to an appropriate ground point on your car’s chassis or engine.
  5. As the O2 sensor’s voltage would go around 0.1 to 1 volts, you better scale the voltmeter in one volt.
  6. Start your car again and check on your connected voltmeter while the vehicle is warming up. If the reading is stagnant even though the car is already warmed, there might be issues with your O2 sensor. You can do another round of tests and stop there.
  7. What you need to do next is to find the vacuum port that’s usually located at the engine top and perform a vacuum leak by opening it. This procedure would fluctuate the O2 sensor’s voltage from 0.1 volts to 0.5 volts as the leak is engaged.  The main idea is to check whether or not your O2 sensor can respond to the low air-fuel mixture created through the procedure.
  8. A propane enrichment test is an alternative procedure if your car doesn’t have a vacuum port or it’s not possible for you to do a vacuum leak.  The O2 sensor’s voltage should rise after you put some propane gas into the air intake. If it doesn’t, the test result informs you of malfunctions in your O2 sensor.  
  9. You can also close the choke lightly which would have caused the voltage of your O2 sensor rapidly rising.

If none of these tests don’t cause your voltage changes on the connected voltmeter reading, then your O2 sensor is malfunctioning, and you’d likely have to replace it.  


What should O2 sensors read on a scanner?

The equilibrium point of the air and fuel mixture in the exhaust range from 14 to 1 in scale, while the O2 sensor read is around 0.45 volts.

However, if it’s a rich fuel mixture, the voltage reading on the oxygen sensor would go up about 0,9 volt, but it would go down to around 0,1 volts if it’s a lean fuel mixture.  The reading on the scanner is essential to determine whether or not your sensor is working fine.

How long can you drive with a bad O2 sensor?

Yes, a car with a bad O2 sensor is still driveable, but you’d have to pay more for fuel due to the inefficiency. You better replace the malfunctioned O2 sensor to prevent it from damaging other components, especially the catalytic converter.

There’s no guarantee how long you can drive the car with a bad O2 sensor until it starts wrecking other parts.

What happens if I unplug my O2 sensor?

O2 sensor’s job is to inform the car’s computer about the oxygen level in the exhaust so the system can either burst more or less fuel in the manifold.

If you unplug your O2, the computer will not obtain the correct air/fuel mixture reading. You’d have to close the sensor’s bung with a fitted plug to keep the gasses inside the pipe before you can drive the car.

However, it’s not recommended to drive your car with an unplugged O2 sensor.

How do I know if my upstream or downstream oxygen sensor is bad?

Both upstream and downstream oxygen sensors share similar signs, including decreasing fuel efficiency, engaged engine light, rotten smell of black smoke coming out from the exhaust, high emission levels, or power surges in the engine.

You can run an emission test with an OBD2 scanner or let it be diagnosed at a reputable auto shop nearby.

Obd2 scanners that are able to test o2 sensore:

Can you trick an O2 sensor?

There are two ways you can trick an O2 sensor. First, you can use a dummy O2 sensor that allows you to bypass the sensor’s reading. Second, you can use an O2 simulator to connect to your car’s computer and set the normal-voltage default.

However, tricking an O2 sensor could be risky and not legal, and that can void your car’s warranty.

O2 sensor that is usually going bad: Whether it’s the upstream or downstream O2 sensors, they can fail for one or another reason. Experts suggest replacing both O2 sensors even if only one is failing, assuming that O2 sensors have the same lifespan. Besides, O2 sensors are not an expensive spare part to buy.

Can bad spark plugs cause O2 sensor code?

Yes, ECM would set the code after receiving a reading from the O2 sensor near the catalytic converter as the bad spark plugs cause misfiring that leaves excess oxygen in the manifold or pipe. As you’re using an OBD2 scanner, you’d see the code on your interface.

Can AutoZone test my O2 sensor? AutoZone provides you with valuable advice and references about replacing faulty or worn O2 sensors. It guides you to check your O2 sensor with an OBD scanner to obtain a reading of whether or not there are problems with the sensors.

As you get the error codes, you can also shop for your O2 sensor replacement there.  

How to Get Rid of the “Check Engine” Light: 4 Techniques

These are four standard techniques to reset the “check engine” light:

  1. Let the light normalized by itself by driving your car for 10-20 minutes
  2. Start and stop the engine three times consecutively
  3. Disconnect the battery for 10-15 minutes and reconnect it
  4. Simply grab a reliable OBD reader to read and clear the engine codes

When should you replace O2 sensors

Common O2 sensors have an average lifespan of 60,000 to 100,000 miles which can be the minimum and the maximum limit of replacing the unit. A replacement is also needed when O2 sensors are failing or malfunctioning even before reaching their lifespan.

Cost to fix oxygen sensors: The actual cost of oxygen sensors would be subject to car models and auto shop’s labor cost. The replacement unit of O2 sensors is around $120 to $250, while the labor cost may start from $100  to $150.

The total cost of O2 sensor replacement can be about $220 to $400.

Final Word

Once you’ve noticed signs of failing or error codes in your OBD II scanner, there are problems with your car’s O2 sensors. You can use our guide on How to use an OBD2 scanner above to confirm the faulty performance.

Multiple factors can lead to a failing O2 sensor in the vehicle’s exhaust system, but you need to respond to it immediately. The straightforward solution of malfunctioning O2 sensors is a unit replacement that would restore the oxygen reading performance. 

Even if your O2 sensors aren’t failing, you should replace them on the recommended mileage.

Related Article: Can You Reset TPMS With Obd2 & How Do You Do That?

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