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How To Diagnose And Fix An Oxygen Sensor Under 30 Mins


Oxygen Sensor Diagnosis

An oxygen sensor, also known as a lambda sensor, measures the amount of oxygen content in the exhaust gases generated by the engine and helps in maintaining a correct air/fuel ratio. The oxygen sensor reads the content of oxygen in the exhaust gas and sends a signal to the Engine Control Unit (ECU) for analysis. If the mixture is lean (more oxygen and less fuel) or rich (more fuel and less oxygen), the ECU will regulate the air/fuel ratio to ensure no oxygen is present in the exhaust gas consequently reducing the emissions.


The oxygen sensor not only reduces the emissions but also prevents your catalytic converter from getting damaged. Few signs a failing oxygen sensor will show are reduced fuel economy, failed emission test, rough engine idling, and smell of rotten eggs from the exhaust. A check engine light along with any of the above symptoms is a possible sign of a problem with your oxygen sensor. The best way to be sure of a faulty oxygen sensor is to identify the Diagnostic Trouble Codes (DTC) stored in the ECU with the help of a Diagnostic Tool. However, a trouble code indicating a bad oxygen sensor doesn't mean it is necessarily the source of the problem. Often a loose or torn vacuum hose or a poor electrical connection can cause the sensor to malfunction. Just replacing the sensor might not solve your problems, therefore making it necessary to test the unit to ensure whether the sensor or something else is causing the issue. 


There are two methods to test your oxygen sensor. First, using an Onboard Diagnostic (OBD) Tool and the other using a multimeter.


Tools Required To Diagnose Your Faulty Oxygen Sensor

1. Test Oxygen Sensors Using An Onboard Diagnostic (OBD) Tool (Under 20 Minutes)

Onboard Diagnostic (OBD) tools are advanced gadgets that provide comprehensive access to live and recorded data that help in troubleshooting vehicle-related problems. Unlike old vehicles, cars today are equipped with more than one oxygen sensor. Usually, there are two oxygen sensors for each bank of cylinders, one before (upstream) and one after (downstream) the catalytic converter. When you retrieve a diagnostic trouble code from the ECU using a diagnostic tool, it will display the particular sensor which is at fault. For example, the diagnostic tool may show Bank 1 Sensor 1 (02B1S1) or Bank 1 Sensor 2 (02B1S2), etc. Sensor 1 or sensor 2 means upstream or downstream sensor respectively. Bank 2 sensors are located on V-type engines that have a separate line/bank of cylinders. To locate the Bank I and Bank II sensors, you can also refer to the service manual, if necessary.


  • To begin the test, first warm up the vehicle for at least 20 minutes so that it reaches the operating temperature. This will allow you to get accurate oxygen sensor readings.

  • Connect the OBD2 scanner through the diagnostic link connector (DLC) of your vehicle. Refer to the vehicle's manual for the exact location of the DLC.

  • Start the engine, select the oxygen sensor testing option from the OBD scanner menu and select the sensor you want to test.

  • For each upstream oxygen sensor, the OBD II tool should show a voltage value fluctuating between 0.10 to 0.90 V (100 mV to 900 mV). If any upstream sensor shows no signal, or the voltage remains the same, the sensor is faulty and needs replacement.

  • The downstream oxygen sensors should show a steady voltage reading of approximately 0.45 volts. Any major fluctuation in the reading of a downstream sensor shows that it is faulty and requires replacement.


2. Test The Oxygen Sensor Using A Multimeter (Under 30 Minutes)

The easiest way to check an oxygen sensor is by using a digital multimeter. If the oxygen sensor on your vehicle has more than two wires, it probably has a heating circuit and a simple resistance test can be performed using a multimeter to check if it is working fine. You can also carry out a comprehensive voltage output test of the oxygen sensor without using a costly tool like a diagnostic scanner. Follow the stepwise procedure to diagnose your faulty oxygen sensor easily.


A. Test The Heating Circuit Of The Oxygen Sensor

  • First, disconnect the sensor by removing the electrical connectors and the oxygen sensor using an oxygen sensor wrench.

  • Then, switch the multimeter on and rotate the dial to put it in the resistance mode. Set the multimeter to a 200 ohms setting.

  • Test the heating circuit of the sensor by connecting one test lead of the multimeter to the heater power pin and the other to the ground pin on the connector. The sensor has four wires with two separate wires having the same color. These wires constitute the heating circuit of the sensor. Follow these two wires leading to the heater power pin and the ground pin on the connector.

  • Once the test leads are connected, the multimeter should show a reading between 2 ohms to 14 ohms. Although the resistance can vary from sensor to sensor, ensure that the reading matches the manufacturer’s specifications.

  • If you see no reading, the heater circuit is broken or faulty and the sensor has to be replaced.


B. Test The Oxygen Sensor

  • Turn on the multimeter and set it at 20 volts.

  • Start the car and let it run for 10-15 minutes until it reaches the operating temperature.

  • Once the car has reached the operating temperature, turn off the engine and connect the red probe of the multimeter to the oxygen sensor's signal wire. On sensors with one to three wires, connect the black probe of the multimeter to a good ground on your engine. If your car has an oxygen sensor with 4 wires, connect the black probe to the sensor's ground wire.

  • Now start the engine and read the sensor voltage signals on the multimeter.

  • The voltage reading should fluctuate between 100 mV to 900 mV (0.10 to 0.90 V). This indicates that the oxygen sensor is working properly.

  • If the voltage reading stays steady and doesn't fluctuate, ensure that the vehicle is warmed up enough. If it still stays steady, there is a problem with the oxygen sensor or there is some issue with the engine.

  • To verify the sensor operation, follow with a lean and rich running test.


C. Test The Oxygen Sensor For A Lean Running Condition

  • Keep the engine running and the multimeter connected to the sensor wires.

  • The first step is to disconnect the Positive Crankcase Ventilation (PCV) hose from the intake manifold to allow more air into the engine making the engine run lean. Refer to the manual if you have difficulty spotting the PCV hose.

  • Check the signal reading on the multimeter. It should show a reading close to 200mV (0.20V). If it doesn't display a reading of around 200mV, the sensor might be faulty and has to be replaced.

  • Attach the PCV hose back to the intake manifold.


D. Test The Oxygen Sensor For A Rich Running Condition

  • With the engine running and the multimeter still connected, disconnect the air filter hose from the air cleaner assembly.

  • Block the opening of the air duct to reduce the amount of air entering the engine. This will make the air/fuel mixture rich.

  • Now, the multimeter should read close to 800 mV (0.8V). If the sensor doesn't display a reading close to 800 mV, it is faulty and needs to be replaced.



Often, the solution to an engine problem is a replacement. Often it's not and can cost you hefty bills if you go on replacing parts without actually getting to its root cause. Maybe there is a loose vacuum hose or a leak in the exhaust manifold gasket causing the oxygen sensor to read high oxygen levels. Or maybe a loose connection is causing it to malfunction and read exhaust gases incorrectly. You can't figure that out until you get your hands dirty and proceed with this oxygen sensor diagnostic procedure. If your sensors respond correctly to these tests, you might have an issue somewhere else in the engine. Carrying out a comprehensive oxygen sensor diagnosis will save you money on unnecessary replacements and help you fix the potential problems that might come up later.