4 Colors of Used Oxygen Sensors and Their Causes

4 Colors of Used Oxygen Sensors and Their Causes – In engines with a fuel injection system, oxygen is the most important part. This is because oxygen is a substance that helps to burn fuel. In addition, oxygen also determines the proper performance of the engine. So, in order to achieve the correct air-fuel ratio, the manufacturer has equipped his machine with a device called an oxygen sensor. The oxygen sensor is located in the exhaust system, more precisely on the exhaust pipe before and after the catalytic converter. The oxygen sensor is often also referred to as the lambda sensor. We can see this lambda value in the exhaust analyzer. to find out more details about lambda, I wrote about it in the article Calculating Lambda on Air Fuel Ratio.

However, before we study the color of the free oxygen sensor and its causes, it’s a good idea to study how the oxygen sensor works on a car engine first.

How Oxygen Sensors Work on Car Engines


The oxygen sensor will generate a voltage according to the amount of oxygen in the exhaust gas it detects. Thus, it will provide real-time feedback on the composition of the fuel and air mixture at that time. Furthermore, the engine management system will correct accordingly the voltage results from the oxygen sensor to improve the air and fuel mixture in each engine speed range. The goal is that the combustion that occurs in the combustion chamber is always close to the ideal position of 14.7: 1 (lambda 1).

Oxygen Sensor Shape

The oxygen sensor has a very different shape from other sensors in a car. Both gasoline cars and diesel cars, both have a similar form of the oxygen sensor. If another sensor is directly connected to the connector, the oxygen sensor has a long enough wire before it connects to the connector.

Colors of Used Oxygen Sensors
The picture above shows the parts of an oxygen sensor on a car in general.

The Oxygen Sensor consists of a battery of ‘galvanic cells’. The sensor also contains two porous platinum electrodes. Not only that, but it also has a ceramic electrolyte (Zirconium Dioxide). The Oxygen Sensor produces a voltage with the smallest range of 100mV (0.1 volts) to the largest of 900mV (0.9 volts). The output voltage depends on the level of oxygen levels in the exhaust gases. The oxygen sensor in operation will compare the oxygen in the atmosphere, usually around 21%, with the amount of oxygen in the exhaust gases.

Oxygen Sensor Assessment Criteria

Before you understand what a rich mix and a lean mix are, it helps you to understand the difference between the two. In a previous article, we discussed the difference between fat and lean mixtures in the fuel system.

  1. Rich Mixture – In this condition means that what happens is that the oxygen entering the combustion chamber is less than 14.7 ( eg 10:1 ). This amount of oxygen will cause a large difference between the oxygen level in the atmosphere and that in the exhaust. This will result in high conductivity between the electrodes. Therefore, the output voltage will also be high around 900mV.
  2. Lean Mixture – In this condition, it means that what happens is that the oxygen entering the combustion chamber is greater than 14.7 (eg 20:1). smaller differences between oxygen levels. This results in less conductivity and a smaller output voltage, typically around 100 mV.
  3. Normal Mixture – The ideal mixture on the engine is about 14.7:1. If this were the case in the engine the output voltage from the oxygen sensor would be around 450mV.
Colors of Used Oxygen Sensors
4 wire oxygen sensor wiring diagram on a car. Image source: Easy Car Electrics

Oxygen sensor features

Here are some criteria for a car engine oxygen sensor (petrol or diesel):

  1. The oxygen sensor has a stainless steel wire. This will provide better resistance to corrosion and thermal stress.
  2. The manufacturer uses gold-plated terminals on the signal and reference connector pins.
  3. Laser-welded dual sensors prevent moisture from entering the sensor/heating elements.
  4. The manufacturer performs a functional quality test on the O2 sensor at 1000 °C.
  5. Manufacturers also test the ceramic thimble pressure at 420 bar to ensure integrity.
  6. Oxygen sensor measuring elements undergo “gas permeation” testing during manufacture.

That’s our explanation of how the oxygen sensor works. Hope you can understand it easily. Thus, the characteristics of the damage that occurs to the oxygen sensor can also be seen from the voltage signal that is issued.

4 Colors of Used Oxygen Sensors and Their Causes

After you understand how the oxygen sensor works on a car engine in general, I will try to discuss the color of the used oxygen sensor in a car. This will be very easy for you to understand because it becomes part of the whole oxygen sensor concept. If you prefer to watch a video on this issue, please watch the following video.

Let’s discuss one by one the problem of the Colors of Used Oxygen Sensors on your car engine.

First – Greenish Used Oxygen Sensor

The color of the first used oxygen sensor is greenish. Some oxygen sensors after we remove them from the holder in the exhaust gas line will have a greenish color. When this happens, usually the engine will also experience problems such as unstable rpm so there will be stinging smoke coming out of the exhaust. Then, why does this happen?

The analysis and what I found are as follows. The green color of the sensor marks is from a cylinder head gasket problem, a bent cylinder head, or a warped cylinder block. These three things are the cause.

Read more: Overheat Engine due to bad cylinder block; Honda City VTEC 2006

Remember the color of the coolant is green – there is red but much more green. If the three sources of the problem above occur, a coolant radiator (green) from the cooling system can enter the combustion chamber. For a while, it will not affect the color of the oxygen sensor. However, in the long term, the coolant vapor will also burn in the combustion chamber. Because water cannot be burned, the water (coolant) will only evaporate along with the exhaust gases through the exhaust. Because the exhaust gases pass through the oxygen sensor, the longer the oxygen sensor color will change. And because the coolant is green, the sensor will turn green.

To solve this instead of replacing the oxygen sensor and done. We really need to replace the oxygen sensor. However, we also have to solve the problem of damaged cylinder head gaskets, repair warped cylinder heads, or even repair cylinder blocks. Thus, the exhaust gas will be normal and the color of the next oxygen sensor will be normal too.

Second – Blackish Used Oxygen Sensor

The second Color of Used Oxygen Sensors is black. Where did the black color on the oxygen sensor come from? This is none other than the oil that burns in the combustion chamber. However, too much fuel enters the combustion chamber, especially diesel, so this can also be the cause.

The oil that enters the combustion chamber can come from various sources. Damage to valve seals, valve guides, worn piston rings, worn cylinders and pistons, damaged cylinder head gaskets, cylinder heads, and warped cylinder blocks are some of the most common causes of oil escape into the combustion chamber. Oil leaks from various sources cause oil to enter the combustion chamber. The oil will burn along with the fuel and air.

Read more: 7 Ways to Overcome Oil Seepage in Car Engines

However, remember that oil is oil, not fuel. So the oil may not burn completely. The remaining oil or oil vapor from the combustion chamber will come out into the free air. Unfortunately, it will pass through the oxygen sensor. Over time, this will cause the oxygen sensor to turn black.

If this is the case, usually the oxygen sensor will be damaged and we must replace it. However, this is not over yet. We have to address the problems I mentioned above as some sources of oil get into the combustion chamber. If it is, then the possibility of the oxygen sensor will be blackish is no longer the case.

Colors of Used Oxygen Sensors
We can see the engine damage from the color of the exhaust oxygen sensor.

Third – Used Oxygen Sensor is a Brown

The third Color of Used Oxygen Sensors is brown. Unlike the first and second, the brownish color comes from the problem of the fuel-air ratio being too rich. How can this happen and how to solve it?

An overly rich fuel-air ratio can stem from a number of causes. Some of them are dirty/damaged injectors, MAP / MAF / IAT sensors have problems, and finally, the ECU is no longer normal. All that is the cause of the fuel to air ratio being too rich. As a result, the fuel does not burn completely and fuel vapor will enter the exhaust channel on the engine.

Read more: 5 Causes of a Damaged ECM or ECU, Anything?

If it happens in a short time then it will have no impact. But if for a long time, the color of the oxygen sensor will turn brown. How to solve this problem?

In such conditions, the oxygen sensor is rarely damaged. Because the contaminants from the fuel are not too dangerous for the oxygen sensor. However, the damage to the injectors, the sensors that I mentioned above, and of course the damage to the ECU must be resolved. Because usually in this condition the exhaust gas will feel pain in the eyes and the smell is very pungent.

Fourth – Reddish Used Oxygen Sensor

The fourth Color of Used Oxygen Sensors is reddish in color. This is the least commonly used oxygen sensor color. But it exists and is quite dangerous. Where did the color come from?

The reddish color of the burn oxygen sensor comes from the additives added to the fuel. There are some people who believe that adding fuel with additives can improve car performance. Some argue that increasing RON is also said to be able to purify fuel. But whatever it is the addition of additives is also widely used.

Read more: The Right Cetane Number for Diesel Cars

We don’t really need to give additives in a car. This is because basically cars have been designed in such a way that they can perform well with the available fuels in a given country. Unfortunately, additives will actually trigger problems in the engine. From injectors to oxygen sensors. Additives will enter the combustion chamber and burn there. The rest of the material that is not completely burned will come out into the exhaust gas channel and will make the oxygen sensor turn reddish in color.

Usually, the damage will occur in the engine oxygen sensor that uses additives. So, we have to replace it. But another thing that is no less important is to stop the use of additives and at the same time clean the combustion chamber and injectors. If we need the fuel filter, we have to clean it as well as the fuel tank, we have to clean it.

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