What Are The Symptoms Of A Bad O2 Sensor?

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what are the symptoms of a bad O2 sensor

Do you feel like your car is not functioning like it used to? Maybe you have a feeling that you’re spending more money on fuel and it was not like that before. Or maybe you smell fuel on your exhaust pipe. Every one of these outcomes has some symptoms behind.

So what can be the signs of a defective O2 sensor? Among these symptoms we can include poor fuel economy, irregular engine function when idling, faulty emission test, engine misfires and catalytic converter issues. On top we can also find the check engine light visible on the dashboard, a fact which is telling that for sure an issue is present.

Symptoms of a bad or faulty oxygen sensor

During a combustion of air and fuel the oxygen it’s burned. The oxygen sensor checks the gas composition that leaves the engine chamber through the manifold.

But we will talk about how the sensor is functioning in the following paragraphs after we find out what are the symptoms of a bad oxygen sensor.

1. Irregular functioning of engine when idling

We can easily observe this symptom when you are stationed in your car. Usually, the RPM when the car is not driving, it’s less than 1000 RPM. But if you feel it’s switching between different values, irregularly, going up and down, then you have a problem.

Although, this can be caused by other factors also, check the oxygen sensor also, as we said, it’s tracking the O2 level in the exhaust gases.

2. Check engine LED highlighted

It is wise to not blame first the O2 sensor for this issue. Although the check engine light can hide multiple things, symptoms of a faulty oxygen sensor is included in them.

Anyway, the best approach will be to first diagnose the issue and then you will know what’s the choice to follow.

As an explanation is that in this case, the engine control unit will try to compensate for the lack or abundance of oxygen the sensor transmits. Also, the engine will not perform correctly and check engine light will turn on to signal the issue.

3. Poor fuel economy

This is probably the most easily detectable symptom because you know how much fuel your car is using. When a larger quantity of fuel is needed then it means that something is wrong.

With this in mind, just imagine that a bad oxygen sensor will not send correct information about the amount of O2 resulted after combustion. In this case, more fuel it’s present which most probably is not going to be used, but still it’s going to be evacuated.

4. Emission test not good

This is also easy to detect if they require you to perform a periodical emission test. Because it can evacuate a lot of fuel, the car will not pass it.

It’s not automatically only the fault of the oxygen sensor, but check if it’s the case. What other things you can find can surprise you.

5. Engine misfires and catalytic converter issues

When the fuel is not burnt completely because of less or too much fuel, problems arise. If there is too little fuel in the engine chamber, this one will cause a significant loss of power and misfiring.

This contrast is derived because of the closed-loop in which the sensor is operating. The engine control unit changes the amount of fuel and is waiting for the response of the O2 sensor. If the response is not accordingly to keep a stoichiometric ratio, the two lean and rich loops of fuel quantity come up.

This will cause hectic behavior in your engine and can cause a lot of damage to the catalytic converter and the engine itself.

What is the oxygen sensor in a car?

Often called “lambda sensor”, the car oxygen sensor, it’s referring to the ratio between the fuel and air. This electronic device will measure how much O2 the air is carrying when it’s exiting the manifold of an engine.

It’s an important part of the fuel injection system in a car and also helps a lot within controlling the pollution that a vehicle is producing.

Like we said above, the oxygen sensor will measure the output of the exhaust of an engine and it’s forming a closed loop with the injection system. This means that we can control how much fuel or air is needed to have proper combustion. Then again, by doing this we can also ensure a proper amount of fuel being burnt, thus diminishing the bad gases that are evacuated in the atmosphere.

A closed-loop, it’s referring to a system in which the engine control unit will change the amount of air and fuel based on the information received from the sensor. To achieve the best mixture for proper combustion and limited emissions, we need good behavior from the sensor.

Can you clean O2 sensors?

If you feel that any of the above symptoms apply to you then you can try to clean the O2 sensor. To ensure a proper result, it’s wise to always use automotive products for cleaning this carbon deposits.

Here, you can use a catalytic converter cleaner or emissions system cleaner along with a canister of the car fuel.

For the first option, use the liquid and then follow the recommendation. Drive until the cleaner you are using is going all the way through the system.

For the second option, it requires dismounting the sensor. Take all the precautionary measures to do this and find the right tooling. The sensor can be a bit tight if we never dismounted it, so you might need some helping fluid.

After you take it off, put it in a container filled with gasoline and leave it overnight. The next day use a brush and gently get rid of all the accumulated deposits. Also, try to get rid of the contaminated gasoline in a specialized center.

Another viable option is to search for a professional carbon cleaning center to give you input on how they can proceed.

How do you clean an O2 sensor without removing it?

This is a viable option for somebody who does not want to get off the sensor. Like we discussed above, to do this there are two options.

One is to try an emissions system cleaner and drive your car until it’s spreading overall. This can have a beneficial effect if the carbon deposits are not that high.

The second option is to visit a professional carbon cleaning service that can offer you their opinion on how can they can help you. This is also indicated in some cases where the outcome is a smoothly working vehicle.

How to test a bad O2 sensor?

First of all, you need to dismount the sensor and check how many wires the oxygen sensor has. There are multiple options here.

If you have only one wire, it means that they use the wire as the signal wire and the ground will be the car’s chassis (where the sensor is mounted). For a two wires oxygen sensor, one is the ground, and the other is the signal. This version is more reliable because you have a separate wire for the grounding.

There are options with three or four wires. This oxygen sensors contain also a heating system, because the sensor will properly work on a temperature of about 600 degrees Celsius. Again we will have 1 wire for the signal (in case of the 3 wires sensor – 2 used for the heating device) and similar 2 wires for the signal in case of the 4 wires one.

The approach will be to check which wires are your signal wires first. Then connect a multimeter and set it to 2V scale DC voltage.

The next step will be to heat the sensor, which is a more tricky step. Once the sensor is heating, you see the voltage going up, until you take away the heating source. Then it should go down quickly. I will leave you a nice video if you want to learn more:


Can you drive your car with a bad oxygen sensor?

Although nothing will happen at the beginning of driving with a bad oxygen sensor, components can suffer from it. If you read above about the functioning and the symptoms of a bad oxygen sensor, you can sense that the first issue will be the fuel economy.

Secondly, each time you start the vehicle it will turn the check engine light on in your dashboard if it’s the case of a faulty O2 sensor.

Thirdly, your engine might not behave as you wish. If losses of power or surges and even misfires do not disturb you, then you can drive your car with a bad oxygen sensor. Although, we both know that these kinds of things can disturb a driver quite a lot.


Either you encounter any of the symptoms of a bad O2 sensor we discussed above or you’re searching for an explanation of the wrong behavior of your car, we define many components in an injection system of a car. For sure a bad oxygen sensor can cause a lot of unwanted issues and you can swap it for a new one to enjoy again a nice and relaxing driving.




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