(O2) Oxygen Sensor – Function – Failure Symptoms – Testing

(O2) Oxygen Sensor - Function - Failure Symptoms - Testing
(O2) Oxygen Sensor - Function - Failure Symptoms - Testing

The (O2) Oxygen Sensor measures the difference between the amount of oxygen in the exhaust gas; and the amount of oxygen in the air.

So, With information from the (O2) Oxygen Sensor and other sources the (ECU) can decide whether the air/fuel ratio of your engine is rich or lean.

Also, The (O2) Oxygen Sensor is not able to measure the air or the fuel entering the engine. Because, the (O2) Oxygen Sensor is located in the exhaust and that where it reads the actual oxygen content in the exhaust.

Depending on the year; make and model of your vehicle you could have anywhere from one to four (O2) Oxygen Sensors. The (ECU) attempts to maintain; a certain air/fuel ratio by interpreting the information gained from the (O2) Oxygen Sensor.

Oxygen Sensor (O2) Comparison
Oxygen Sensor (O2) Comparison

So, The ideal ratio for oxygen and gasoline is 14.7:1; which slightly varies depending on different types of fuel. Both the rich and lean mixtures are bad for your car; as well as for the environment.

How Does The (ECU) Read This Information

The information sent to the (ECU) is in the form of a voltage above or below a preset amount. The base voltage is approximately 0.45 V (450 mV) DC.; keeping the air and fuel mixture at the optimal ratio.

A voltage output lower than the base amount around  0.2 V (200 mV) DC.; would indicate a lean mixture.

A voltage output higher than the base amount around  0.8 V (800 mV) DC.; would indicate a rich mixture.

So, Having this information in real time helps determine whether the air/fuel ratio is rich or lean. Finally, If your (O2) Oxygen Sensor fails to function properly; your engine management computer cannot determine the air to fuel ratio. Therefore, the engine is forced to guess how much fuel to use; resulting in a polluted engine and a poorly functioning vehicle.

(O2) Oxygen Sensor Failure Symptoms;

  • Poor gas mileage
  • Loss of power
  • Black smoke from tailpipe
  • Emissions test failure
  • Rough idle
  • Hesitation or stalling
  • Check engine light comes on

Trouble Codes

It’s true that many of these symptoms could be caused by various problems; but the trouble code from the (O2) Oxygen Sensor will narrow it down quickly.

A trouble code that points to an (O2) oxygen sensor (such as p0420, p0135, p0141, or others); is only the first step in your diagnosis of the problem. It turns out that most of the issues that set (O2) oxygen sensor codes; are not a result of a bad sensor.

Compare Oxygen Sensors
Compare Oxygen Sensors

So, What happens to Oxygen (O2) Sensor over time is that they tend to become fouled with carbon and sooty deposits; or the element just simply erodes and wears down; like the electrode on a spark plug. They can also become fouled if any silicone from grease or lubricants makes its way to the exhaust stream; or if any oil or coolant make their way to the combustion chambers. Finally, If the oxygen sensor is too worn out; it will lag in response time or can just stop working altogether.

What Can Cause The (O2) Oxygen Sensor To Fail;

So, Over time, your (O2) Oxygen Sensor can become caked with byproducts of combustion like sulfur, lead, fuel additives, and oil ash. As a result, This keeps your sensors from sending signals to your engine’s computer. Also, Using fuel that isn’t recommended for your vehicle or low-quality fuel can make your Oxygen (O2) Sensor fail faster.

(O2) Sensor Failure Can Cause Catalytic Converter Meltdown

So, An oxygen sensor failure can lead to incorrect readings of exhaust gasses. As a result, The faulty sensor can cause a too rich or too lean condition. Too rich and the catalyst can melt down. While, Too lean and the converter is unable to convert the hydrocarbons into safe elements; and may not pass a state inspection.

Catalytic Converter
Catalytic Converter

So, The fuel that powers your vehicle is meant to burn in the combustion chamber only. Any fuel that leaves the combustion chamber unburned; will enter the exhaust system and light-off when it reaches the catalytic converter. As a result, This can super-heat the converter far above normal operating conditions and cause a Meltdown.

Other Possible Issues To check First

Check For Vacuum Leaks

So, If your engine has a vacuum leak; the air/fuel ratio in your engine will be higher than 14.7:1, also called a “lean” mixture. Consequently, This ratio means that there is too much air in your engine; and as a result; the engine will run poorly or not at all. Finally, What is interesting about a vacuum leak is it can look like something else.

Vacuum Leak
Vacuum Leak

The air/fuel ratio is very important in the proper workings of an engine. So, The proper amount of air has to be present; or else combustion efforts are greatly affected. Also, a leak condition can result in air that is not properly measured entering into the engine. Finally, That upsets the balance and the result can be the engine having some difficulties.

Check (EGR) Valve

So, A stuck open (EGR) valve will create a lack of oxygen in the exhaust; since the recirculating exhaust has all its oxygen already burnt. Furthermore, The (ECM) sometimes uses the Oxygen (O2) Sensor to check for proper (EGR) operation and sets a code if necessary.

(EGR) Valve
(EGR) Valve

So, be aware of the fact that a vehicle might be running lean; because the (ECM) sees a rich Oxygen (O2) Sensor signal; due to a defective (stuck open) (EGR) valve. Since the (ECM) sees a rich signal; it will try to correct with a lean command and try to lower the oxygen sensor high voltage signal.

Testing (O2) Oxygen Sensor Voltage Signals

  • Start the engine and check the sensor voltage signals on your voltmeter. The sensor voltage should cycle or fluctuate within the 100 mV-900 mV (0.10 to 0.90V) range. Hence, This means the sensor is operating properly.
  • If the Oxygen (O2) Sensor only produces a low or high voltage signal, either you have an engine performance issue or the Oxygen (O2) Sensor stopped working. Finally, To verify sensor operation, conduct the next two tests.

Digital Multimeter
Digital Multimeter

Test the  (O2) Oxygen Sensor Response to a Lean Fuel Condition

  1. First, disconnect the hose from the positive crankcase ventilation (PCV) valve leading to the intake manifold. This will allow more air to enter the engine. If you need to locate the (PCV) valve; consult your vehicle service manual.
  2. Check the sensor’s signal voltmeter reading. An oxygen sensor interprets an increase in oxygen as a fuel lean condition; emitting a signal close to 200mV (0.20 V). If the sensor doesn’t respond accordingly or takes time to respond; the sensor isn’t working properly.

Test the (O2) Oxygen Sensor Response to a Rich Fuel Condition

  1. Next, disconnect the plastic duct from the air cleaner assembly on your vehicle.
  2. Block the duct opening leading to the engine with a clean rag. This will reduce the amount of air going into the engine.
  3. Check the sensor’s signal voltmeter reading. An oxygen sensor interprets a decrease in oxygen as a rich fuel condition; emitting a signal close to 800mV (0.80 V). If the sensor doesn’t respond accordingly or takes time to respond; the sensor isn’t working properly.

If the Oxygen (O2) Sensor in your vehicle responded correctly to your tests; you might have a problem with another component affecting fuel efficiency.

So, As you can see testing is way cheaper than just replacing parts.

There are many different types of (O2) Oxygen Sensor and testing methods. So, I will just supply Links from our PDF Library.


Walker Products Oxygen Sensor Training Guide

Testing the Oxygen Sensor

DENSO Oxygen Sensor Diagnosis

DENSO Lambda-Oxygen Sensor

Bosch Testing of Oxygen-Lambda Sensors

So, If you have any trouble opening these files; you may have to download the PDF Files Reader Here.


Oxygen Sensor (O2) Installed
Oxygen Sensor (O2) Installed

So, One of the most important sensors in modern cars is the oxygen sensor. Also known as the (O2) sensor because (O2) is the chemical formula for oxygen; the oxygen sensor monitors how much unburned oxygen is present in the exhaust as the exhaust exits the engine.

So, By monitoring oxygen levels; the sensor provides a means of measuring fuel mixture. Finally, Knowing the ratio of fuel to air allows your vehicle’s engine to make any necessary changes; to ensure that your car runs like it should.

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