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How to Test Your O2 Sensor With an OBD2 Scan Tool

Over 50 percent of smog or emission test failures are caused by malfunctioned O2 sensors. A bad oxygen sensor can literally drain your pocket due to the negative impact it can have on your vehicle. Therefore, it is vital that you know how to test your oxygen sensor to determine whether it is working well or is faulty.

how to test your O2 Sensor
How to Test O2 Sensor With A Scanner

Bonus: NEXPEAK NX501: O2 Evap I/M Readiness OBD2 Scanner with Mode 6 Review 

How to Test O2 Sensor Using an OBD2 Scan Tool

Step 1: Connect the OBD2 Scanner to your car

With an OBDII scan tool connected to your car, note the O2 sensor voltage with the Key On Engine Off and with the Engine Running. You should notice as the vehicle is being driven that the O2 sensor voltage should fluctuate, typically between zero and 1volt. For the first 10 minutes, the longer you drive it, the more it will fluctuate. On newer air/fuel ratio sensors their operation varies between manufacturers. Some operate with varying voltage while others produce a varying amperage signal.

Step 2: Notice the fluctuation of the voltage of O2 sensors

Along with voltage, you should notice how quickly it fluctuates between a low voltage (typically less than .5v) to a higher voltage (more than .5v). This fluctuating or toggling will normally occur between 2 and 5 times a second. If the voltage constantly stays below .5 volts then a lean exhaust code would be setting. Conversely, if the voltage stays high, above .5 volts, then a rich exhaust code would be setting.

Step 3: Determine the code type

Determine the type of code that set. 

If the code points to a lean running engine, go to step 4.

If the code points to a rich running engine, proceed to step 5.

Step 4: Test an O2 sensor – LEAN running engine

First, obtain a propane bottle and attach a valve to the bottle. Attach a hose to the valve and feed the hose into the air intake (open the air cleaner housing and insert the hose into the tube headed away from the air filter housing. Slowly open valve while monitoring the O2 voltage on your scan tool. The voltage must rise above .8 volts. If the voltage readings are attained, then the O2 sensor is not your likely culprit. Go back over engine basics to determine if there are any conditions that could cause the engine to run rich (high fuel pressure, vacuum not present at the MAP (Manifold Absolute Pressure) Sensor, if equipped). If the voltages are not attained, the most likely cause for the readings is a failed O2 sensor. If the mileage is high or if the O2 sensor is old and due for replacement, this would be the most likely cause of failure. If the O2 sensor is fairly new, more diagnostics would be indicated, such as leaking coolant into the exhaust or possibly the wrong RTV sealant was used during a prior engine gasket replacement.

Step 5: Test an O2 sensor – RICH running engine

First, disconnect a small vacuum hose from the engine (large enough to cause the engine to run slightly rough, not large enough to cause the engine to die). Monitor O2 voltage on the scan tool. You should notice the voltage drop below .2 volts and stay low.  If the voltage readings are attained, then the O2 sensor is not your likely culprit. Go back over engine basics to determine if there are any conditions that could cause the engine to run lean (low fuel pressure, intake manifold/exhaust manifold leaks, leaks around O2 sensor, etc). If the voltages are not attained, the most likely cause for the readings is a failed O2 sensor. If the mileage is high or if the O2 sensor is old and due for replacement, this would be the most likely cause of failure. If the O2 sensor is fairly new, more diagnostics would be indicated, such as leaking coolant into the exhaust or possibly the wrong RTV sealant was used during a prior engine gasket replacement.

O2 Sensor Codes Interpretation

O2 sensor codes that this information could be useful in diagnosing

P0130 02 Sensor Circuit Malfunction (Bank I Sensor 1) 

P0131 02 Sensor Circuit Low Voltage (Bank I Sensor I) 

P0132 02 Sensor Circuit High Voltage (Bank I Sensor 1) 

P0133 02 Sensor Circuit Slow Response (Bank 1 Sensor 1) 

P0134 02 Sensor Circuit No Activity Detected (Bank I Sensor 1) 

P014C O2 Sensor Slow Response – Rich to Lean (Bank 1 Sensor 1) 

P014D O2 Sensor Slow Response – Lean to Rich (Bank 1 Sensor 1) 

P014E O2 Sensor Slow Response – Rich to Lean (Bank 2 Sensor 1) 

P014F O2 Sensor Slow Response – Lean to Rich (Bank 2 Sensor 1) 

P0150 02 Sensor Circuit Malfunction (Bank 2 Sensor 1) 

P0151 02 Sensor Circuit Low Voltage (Bank 2 Sensor 1) 

P0152 02 Sensor Circuit High Voltage (Bank 2 Sensor 1) 

P0153 02 Sensor Circuit Slow Response (Bank 2 Sensor 1) 

P0154 02 Sensor Circuit No Activity Detected (Bank 2 Sensor 1) 

P015A O2 Sensor Delayed Response – Rich to Lean (Bank 1 Sensor 1) 

P015B O2 Sensor Delayed Response – Lean to Rich (Bank 1 Sensor 1) 

P015C O2 Sensor Delayed Response – Rich to Lean (Bank 2 Sensor 1) 

P015D O2 Sensor Delayed Response – Lean to Rich (Bank 2 Sensor 1) 

P0171 / P0174 O2 Sensor has detected a Lean Exhaust Bank 1 / Bank 2

P0172 / P0175 O2 Sensor has detected a Rich Exhaust Bank 1 / Bank 2

To know exactly what each code means, use the DTC Look-up Library of your scan tool or go to OBD2 Codes, search the codes you have on the “Search Box”. You will find the Meaning, Symptoms, Causes and Diagnostic Steps of the codes you have.

O2 sensor: What is it for?

An oxygen sensor is located in the exhaust stream. It is used to monitor in real time the ratio of air/fuel to determine whether it is lean or rich. This is done by measuring the amount of oxygen in the liquid or gas passing through the exhaust manifold.

Many people misunderstand the measuring process. The sensor does not measure the concentration of oxygen. Rather, it measures the difference between oxygen in the air and oxygen in the exhaust gas.

Apart from determining the air/fuel ratio, an o2 sensor can also determine whether the catalytic converter is working well. The sensor can adjust the air/fuel ratio to ensure the catalytic converter is working efficiently.

Once the ratio is determined, a voltage is created and it flows through the o2 sensor, sending the reading to the CPU. The CPU then adjusts the ratio according to what is appropriate. This reduces exhaust emissions thus contributing to a healthier environment.

Tampering with the o2 sensor signal could negatively affect emissions control since it controls emissions. Additionally, it can also cause damage to the vehicle. For example, a faulty o2 sensor can damage the catalytic converter.

Types of O2 Sensors

Heated oxygen sensors are found in modern vehicles, from the 1990 models up to present.
Credit: commons.wikimedia.org

Oxygen sensors can either be heated or unheated. However, in modern times, heated oxygen sensors are more common in vehicles.

Unheated oxygen sensors are found in old vehicles, up to the 1990 models. These sensors need to be replaced more often compared to heated oxygen sensors. It is advised that the sensors be replaced after every 30,000 to 50,000 miles.

Heated oxygen sensors are found in modern vehicles, from the 1990 models up to present. These sensors are more durable and should be replaced after every 60,000 to 90,000 miles.

Since oxygen sensors are exposed to harsh conditions, they should be replaced at the appropriate time to avoid problems.

Bad O2 Sensor Symptoms

There are various signs that hint your vehicle has a problem with its oxygen sensor. Some of these signs include:

1) Foul Exhaust Odor

When the o2 sensor has malfunctioned, a strong, unpleasant odor will emanate from the exhaust. If the problem is not dealt with immediately, the smell could even spread to the vehicle’s interior. This would be such an embarrassing situation.

2) The Check Engine Light is On

The Check Engine Light is an indication that there is a problem with the vehicle. Although it does not specifically mean that the problem lies with the oxygen sensor, there is a possibility the o2 sensor has a problem. Always get your vehicle checked immediately you notice the light is on.

3) Decreased Vehicle Performance

Your vehicle will generally reduce its performance in various areas. You may experience engine stalling and weak acceleration. Additionally, you may notice the engine runs roughly or irregularly even when it is idle.

4) Bad Gas Mileage

Have you noticed you fill up your gas tank more often than you usually do? This may be due to bad gas mileage. When the oxygen sensor fails to work effectively, the mixture of fuel and air will be rich.

A rich mixture of fuel and air means more gas is consumed during the process of internal combustion. This means a higher fuel consumption of gas and digging deeper into your pockets.

The Bottom Line

Never ignore any signs of failure of the oxygen sensor. The earlier the problem is diagnosed the better.

Solving o2 sensor problems early will enable you to save financially since little to no damage would have been done to the vehicle. Additionally, you will save on the amount of gas you use while driving.

If you find it difficult to use the OBD scan tool, take your car to a good mechanic to check it out.

Read more: Best OBD2 Scanners Complete Buying Guide

Randy Worner

Randy Worner, from Conroe, Texas, has been working on cars and trucks for almost 45 years. He is currently a Technical Service Support Specialist at Cummins Inc. Randy has had 36 years' experience teaching Automotive / Diesel Technology classes for UTI, Snap On Tools, Chrysler, Pepboys, Lone Star College, NAPA and TBC Corporation.

Web: http://obdadvisor.comEmail: [email protected]
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