Why Do Oxygen Sensor Codes Keep Coming Back? (Cause and Fix)

Are you wondering why Oxygen sensor codes keep coming back? Are you confused as to what it means? Bothered about how to fix this?

Then you are just where you ought to be because we have you all covered. We have analyzed the possible reasons why Oxygen sensor codes keep coming back, how to figure out the particular source of the problem and how to fix it.

Oxygen sensors help regulate the fuel-air mixture. The Oxygen sensor codes indicate an anomaly to this ratio, while the sensor triggers a response to balance it out.

So if the Oxygen sensor codes keep coming back, it could be that the faulty oxygen sensor was not replaced properly or other issues causing it, like bad wiring or even a faulty spark plug.

Reasons Oxygen Sensor Codes Keep Coming Back

Oxygen Sensor Codes Keep Coming Back

1. The Error Code Didn’t Reset

If the 02 sensor code keeps returning and you have confirmed the codes are similar, it may be as a result of the computer not recognizing that the issue has been resolved. To correct this, try these:

  • Drive some distance to enable the computer to calibrate properly and realize that you have resolved the issue that led to the codes.
  • Disconnect the battery for 30 minutes; however, understand that removing your battery will also affect some other data. (Newer cars have non-volatile storage for keeping data such as error codes for emissions, so unplugging the battery might not change anything)

In a case where you are sure this is the issue, readjust the codes using an OBD2 scanner.

2. Your Oxygen Sensor Wasn’t At Fault

It has been repeatedly said that if the oxygen sensor codes keep coming back, it shows that the sensor was not the problem, and replacing it was a wrong move.

To avoid this kind of situation, take a look at the chart we created below to know the meaning of those oxygen sensor codes and what the likely origins are.

3. You Replaced The Wrong Oxygen Sensor

Modern cars usually have two oxygen sensors. The job of the first sensor is to change the fuel-air mixture, while the other ensures the catalytic converter and the first oxygen sensor are working well.

There is a likelihood the second O2 sensor is faulty, thereby tricking the computer into processing that the first oxygen sensor is bad. The wiring may also be the issue at hand.

Though unlikely, but there is a possibility that the computer has been tricked by a different sensor that’s nullifying the input signal of the oxygen sensor.

Also Read: 10 Bad O2 Sensor Symptoms (Causes and Solutions)

How to Test an Oxygen Sensor

Oxygen Sensor Codes Keep Coming Back

You may need to find out whether your O2 sensor is responsible for the error codes returning. A test can be carried out using an OBD2 scanner or multimeter.

1. Remove the Oxygen Sensor

Based on the type of error code you received, it may be necessary to take out the O2 sensor in different positions because more than one oxygen sensors are present. The job of the first sensor is to change the fuel-air mixture, while that of the other ensures the first oxygen sensor and catalytic converter are working well.

Some cars have as many as four oxygen sensors. The reason for this is that any car produced in or after 1996 has a second O2 sensor positioned beneath the catalytic converter in order to keep track of its efficiency and show if there is an obstruction or if it is spoilt.

Air Fuel Oxygen Sensor

If you need to remove the O2 sensor, an oxygen sensor socket that can slide on top of the cheater bar will be required. This device will provide an advantage in loosening the sensor.

  • Go beneath the car and find the electrical network.
  • Disconnect the socket
  • Loosen the oxygen sensor using the cheater bar with an O2 sensor socket
  • With hand, unscrew the sensor
  • Done

Other Location

If it’s difficult to locate the oxygen sensor, a search on google or consulting your car’s owner’s manual will help you find the location of the oxygen sensor. If it’s located in a strange spot, there is no cause for alarm. It shouldn’t be any different from the steps we’ve listed above, and it might even be much easier and more creative.

Also Read: How to Remove O2 Sensor without Socket (Step by Step)

2. Determine Which Oxygen Sensor You Have

Before embarking on the testing process, you should know that your O2 sensor may consist of 1 or up to 4 wires:

1 Wire: It has only one wire which is the signal wire, grounded by the exhaust pipe within the chassis. This relays the voltage data to the ECU.

2 Wires: This has one signal wire which is a sensor ground wire. This is more dependable than attaching it with the exhaust pipe chassis.

3 Wires: This consist of a signal wire grounded by the exhaust pipe within the chassis and an extra wire that serves as the heater, that helps the sensor attain the normal temperature for its operation quickly, as there is no production of voltage by the oxygen sensor until it reaches a temperature of 600 degrees.

4 Wires: This consist of a signal wire, one ground wire, and two heating wires.

Conventionally, the signal wire is black in color, the heater wire is white, and the ground is grey.

3. Test the Oxygen Sensor

We would use an oxygen sensor with four wires to explain. Since this oxygen sensor has a heater, you have to determine the internal heater resistance.

  1. Put the multimeter on
  2. Ensure the setting in on Ohms, and put it to 200.
  3. Examine how the oxygen sensor would react to a case of lean fuel consumption. Pull the hose out of the positive crankcase ventilation (PCV) valve that is usually on the cover of the valve. This is so that more air enters the engine. The voltmeter reading at this point should be about 200mV. Failure of the voltmeter to respond simply means the oxygen sensor isn’t working well.
  4. Plug the PVC hose, and examine how the oxygen sensor would respond to a case of rich fuel consumption. To carry this out, unplug the plastic hose from the air cleaner assembly. Clog the opening in the hose connection with a cloth to limit the air entering the engine.
  5. Observe the voltmeter.The voltmeter reading should be about 800mV due to the reduced oxygen going into the engine. The sensor is not working well if this does not occur.
  6. Plug the hose back into the air cleaner.
  7. If the oxygen sensor’s response to this test was satisfactory, the Fault is from some other component. The problem could be the ignition network, a vacuum leak, or something related. However, if the sensor fails to respond, it’s faulty and needs to be changed.

Watch this video for more tips on how to test an O2 sensor

Frequently Asked Questions – Oxygen Sensor Codes Keep Coming Back

Why does my oxygen sensor keep failing?

Why do oxygen sensors fail? First, the O2 sensor can get contaminated since it’s in the line of the stream from the exhaust. Some causes of this contamination are overly rich fuel mixture or, as in the case of an old engine, oil blow-by, and burning of the engine coolant in the combustion chamber, resulting from a gasket leakage.

What can cause a false oxygen sensor code?

Misfire from the engine, exhaust valve leakage, or leakage from the exhaust manifold gasket, which lets air enter the exhaust, can result in a false code. However, the most frequent reason oxygen sensor fails is contamination, especially if it occurs prematurely.

Why is my check engine light still on after changing the oxygen sensor?

Changing an oxygen sensor simply because you received an oxygen sensor code or the light is turned on does not imply that the new sensor may solve the issue. The Check engine light can be on for various reasons. Having code for a particular part does not mean that part is faulty.

How long does it take for an oxygen sensor to reset?

Put the ignition in the start point, but don’t turn the engine on. Instead, place the fuse back in after about five minutes. The ECU is fixed if the check engine lamp blinks and goes off.

Can bad spark plugs cause O2 sensor code?

A bad spark plug can make the car’s internal processor interpret wrongly and activate the oxygen sensor code. So it’s risky to drive with a bad spark plug.

Is it OK to drive with a faulty O2 sensor?

You can move your car when the oxygen sensor is bad or faulty, but you need to replace it as soon as possible. Ignoring it would make you spend more money than usual on fuel daily, and it may damage your catalytic converter, forcing you to get a new one.

Will the P0420 code clear itself?

P0420 code is a light nag code. Hence, you shouldn’t be bothered much by it. So press cancel and get on with the journey.

Can a misfire cause an oxygen sensor code?

A misfire can cause an oxygen sensor code, though it may not be directly. The misfire may be caused by fuel delivery issues, a mass airflow sensor, or an oxygen sensor. If misfires are not peculiar to a cylinder, there is a high tendency that your engine is not receiving enough air or fuel to function well.

Can an oxygen sensor fix itself?

Oxygen sensors cannot be repaired and hence has to be changed. Some sources may teach you to clean up the sensor, but this will only extend the life for a little while.

Conclusion – Oxygen Sensor Codes Keep Coming Back

You should be able to tell if the oxygen sensor is faulty or probably the problem is something else after running all the tests.

Once the oxygen sensor is faulty, it should be taken out. You can do this yourself. If you are uncertain, visit a car expert. Take to heart that tackling the issue early on would prevent more serious problems like replacing the catalytic converter and save you some money.

Scott Greene is a seasoned automotive technician for over 5 years and has been deep into advanced automotive diagnostics for a couple of years. He Loves writing about Automotive Diagnostics and Repair, Trouble codes, Buyer guides for various car parts and accessories, and lots more. 

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