Home Diagnosis and TroubleshootingOBD Diagnostic Error Codes P0405 – Exhaust Gas Recirculation Sensor A Circuit Low (Fixes)

P0405 – Exhaust Gas Recirculation Sensor A Circuit Low (Fixes)

by Jordan Harris

Have you gotten the check engine light and diagnosed the P0405 code? Well, if that is the case, then you are at the right place because, in this article, there will be a lot to cover.

Car problems are very common. Especially if you own a car that is more than 5 years old or a car that has a ton of miles. So, components can get dirty with debris, electronic components can malfunction, and whatnot. Car ownership can become a complete nightmare.

And frankly, when it comes to this, you have to be prepared if you don’t want to be ripped off by sketchy mechanics that only look for a way to rip you off and take your money. So, being here is a good step by your side since you will be able to learn about this problem and how to solve it.

And frankly, this is why we are here, to educate folks who want to wrench on their cars. And also, there is no better feeling than feeling satisfied once you solved a problem with your car. That’s one of the things that made me a successful mechanic. So, if solving puzzles is one of your traits, you will definitely going to love this.

What we will cover in this article? Well, first we are going to learn what is car diagnostics and we will get familiar with the tools that you will need. Then we will cover what the EGR valve is and the EGR sensor location. Once we clear that, we will dive into the causes, symptoms, and diagnostics of the P0405 code.

Car Diagnostics

Now let’s cover the first topic for today. And this is the topic of car diagnostics. We need to learn what is car diagnostics before we dive into more complex problems like the P0405 code. Modern cars if you didn’t know are nothing like cars that were produced 30 or 40 years ago.

In modern cars, everything is computerized and controlled by the ECM or PCM (powertrain control module) as some people call it. The PCM is the car computer. This computer is basically the brain of the car. So, this computer monitors everything that happens with your car. There are thousands of processes that are tracked constantly.

The PCM is constantly testing the sensors and other components for faults. And when a fault occurs. The PCM will trigger the check engine light. And in this case, the P0405 code on your car.


Then, you need to troubleshoot the problem and determine what is causing it. The description of the code is a good starting point. And then you should move on and think logically and see if there are other codes concerning the system or a similar system that is connected to this one.

Usually, car codes are triggered in a chain reaction. So, if you have an EGR code like this, you can also expect a code concerning a misfire or a similar problem. You get the idea.

And then in order to troubleshoot the problem even more thoroughly, you will need a tool known as a multimeter. With this tool, you will be able to measure electric properties. Like resistance, voltage, and continuity.

So, the OBD2 scanner and the multimeter are pretty crucial when it comes to modern diagnostics. But how to tackle the P0405? More about that, we will cover it in a bit.

EGR Valve

Now before we further elaborate more about the P0405 code, let’s first learn more about the EGR valve and its function in the car. So, what is an EGR valve?

Well, the EGR valve is not something new. This was a component invented in the early 70s for the purpose of reducing NOx particles.

The EGR abbreviation stands for exhaust gas recirculation, this means that the valve recirculates some of the exhaust gases from the exhaust manifold into the intake.

NOx particles are produced by the high-temperature mixture of oxygen and atmospheric nitrogen. So, this EGR valve instead of letting these gases into the atmosphere, is recirculating them again into the intake.

And not only that this reduces emissions. But also helps in improving the fuel economy. What is worth noting is that not all gasoline engines have EGR valves since these valves are not essential on some engines.

But all diesel engines have them since diesel creates a lot of NOx particles. And you need to have one of these valves to cut the emissions of the car.

What is worth noting is that old-school EGR valves are vacuum controlled. There is a diaphragm that opens and closes depending on the vacuum. While newer cars, tend to be electronically controlled by a special sensor. And more about the sensor, we are going to cover next before we explain the P0405 code.

EGR Position Sensor

Now let’s cover the EGR position sensor before we learn more about the P0405 code. What this sensor is?

The EGR position sensor is a simple sensor that determines the position of the EGR valve. And this device sends the data to the PCM to determine the position of the valve and how open it is. As we noted previously, modern EGR valves are electronically controlled and also implement some really interesting tech inside.

Inside the EGR there is a coil and when this coil gets a signal from the PCM it gets energized and basically moves the potentiometer up and down. A pretty ingenious process we must admit.

Very different compared to the old-school EGR valves where the EGR valve opened based on the vacuum that was created inside of the lines. And these EGR valves did more bad than good.

They often get stuck and created a ton of carbon buildup inside that caused the EGR valve basically to fail. Newer electronic valves like in our case are also susceptible to clogging up but to a lesser extent, we must admit. But where is this sensor located?

Well, let’s cover the location of this sensor next before we dive into elaborating on the meaning of the P0405 code.

EGR Position Sensor Location

Now let’s take a look at the position of this sensor before we learn more about the P0405 code and what it represents. Where you can locate this sensor?

This sensor is easy to find because it is located on top of the EGR valve. The sensor basically sits above the potentiometer that we mentioned.

How you will notice is really simple. The EGR valve housing is made out of metal. While the sensor and the connector on top of the sensor are made out of black plastic.

If you still cannot locate it, just browse the web with the name of your engine + the phrase “EGR position sensor location”, or “EGR valve location”. In the results, you will notice a ton of different results and many of them will help you out finding the exact location of the sensor.

Now let’s move on to the code P0405 and understand a bit more about its meaning and whether the code is serious or not.

Exhaust Gas Recirculation Sensor A Circuit Low

So, we covered the EGR position sensor and its location. Now we can focus on our problem with the code P0405. And once we learn the definition, you will see why we had to cover all these themes. So, what is the definition of P0405?

The definition of the P0405 code is “P0405 – Exhaust Gas Recirculation Sensor A Circuit Low”. But what does this means?

Well, this is a code that indicates that the voltage that this sensor is sending to the PCM is low. More specifically circuit A.

To further understand this let’s learn the diagram for this sensor. This sensor is a 3-wire sensor. This means that there is one ground wire and one 5v supply wire that goes from the PCM. And one signal wire that goes from the sensor to the PCM.

And in our case, the signal that goes from the sensor to the PCM is registering low voltage. Which shouldn’t be the case. So, the PCM doesn’t have data to work with and the performance of the EGR valve is at stake because, without this sensor, the PCM will not be able to determine the position and find out how open this valve really is.

And in this case, you will get the check engine light, as well as some symptoms that we are going to further elaborate in the following chapters. Now let’s move on and learn whether the P0405 code is serious or not.

Is This P0405 Code Serious

Now let’s take a look at how serious is this P0405 code on your car. Should you be very worried because of this problem?

Well, yes, the EGR valve is really important, even though it is an emissions device. This device basically prevents your engine from running well if it’s not in a good condition. The same applies to any other EGR-related OBD diagnostics trouble codes, including P0401 and P0404.

This is why we think that you should strive to repair the problem. So, what are the probable causes for this problem? Well, let’s further elaborate on them in the following chapters in detail and see what you can do about it.

P0405 Causes

Now let’s first cover the causes of this problem and understand more about what could cause an issue like the P0405 code.

There could be a few things that you should be aware of and in the following chapters, we are going to cover all of them in detail. So, let’s get into it.

1. Clogged EGR Valve

Now let’s begin discussing the causes of the P0405 code. One of the primary causes of this problem is a simple clogged EGR valve.

As we noted, the EGR valve is regulating the exhaust gases and a lot of gas is going through this valve. So, it is a very common thing for this valve to get clogged.

And once the EGR valve gets clogged, the sensor will be basically stuck and not be able to measure the position with the EGR position sensor.

This is why we strongly recommend removing the EGR valve and checking out if the valve opens and closes correctly.

If it’s full of carbon, you might want to try to clean it. Or if the carbon buildup is too large, you will have to replace the EGR valve with a new one, and consider the EGR valve replacement cost. Now let’s move on to the next causes for the P0405 code.

2. Internal Malfunction In The Sensor

The second most common cause for the P0405 code is an internal malfunction of the EGR position sensor.

This sensor can also fail internally and cause a problem such as this. And this is also a very common cause because the sensor is an electronic device. And as we all know, electronics break sooner or later.

So, if the sensor shorts out on the inside, you will have to replace it with a new one. In some cases, you will also have to replace the whole EGR valve since this sensor comes as a whole with the EGR valve.

You can test the sensors with a multimeter of course to check for a good measure if the problem lies in the sensor or if it is triggered by something else. But more about that, we are going to elaborate later on in the article. Now let’s take a look at the P0405 code and see which is the next cause that can trigger this code.

3. Bad Wiring

The next probable cause for the P0405 code and the check engine light is the bad wiring. As you probably know, there is wiring that goes from the wiring harness to the EGR valve position sensor.

This wiring has three wires, and one of them is the signal wire which is really important to us. So, if there is a problem with the signal wire, for example, if it is cut in two or the insulation is damaged the situation can trigger a code such as the P0405 or a similar one.

So, when it comes to solving this problem we would recommend that you do your investigation and check the wires.

If the wiring is bad, rewire it properly and insulate the wires well. This will hopefully solve the problem with your car and the code P0405. Now let’s move on to the last cause for the P0405 code on your car.

4. Bad Connector

The last cause of the P0405 code is the bad connector. There is a connector that is plugged into the sensor.

This connector is often located on top of the EGR valve. And what can happen is that the connections can get damaged.

There could be rust buildup on the pins and this will cause the connector to lose the connection from the signal wire and trigger this code.

So, you should also check the connector and the condition of the pins. If some of them are damaged, clean them up with sandpaper and make sure that they make good contact. But what about the symptoms of this code? Well, let’s elaborate on them in the following chapter.

P0405 Symptoms

So, we learned the definition, as well as the causes of the P0405 code. Now let’s move on and learn more about the symptoms of a bad EGR valve that are also very common with this code on your car. There are a few of them that are worth your attention.

The biggest symptom is probably the check engine light. Since there wasn’t a check engine light, you wouldn’t be able to be here. So, this is the obvious symptom associated with this type of code.

But there are also other symptoms associated with the problem. If the EGR valve doesn’t work, your car could go into limp mode and you will experience reduced power.


So, you will not be able to push the car after a certain RPM. And this is very common on cars that have their EGR valves completely blocked off or the EGR position sensor failed. So, beware of this issue since the EGR valve plays a big role in the internal combustion engine.

Also, the EGR valve improves fuel economy. So, if it doesn’t work well, you will also experience a reduction in fuel economy. The fuel economy will drop significantly and you will be able to feel this because it will definitely hurt your wallet.

Another symptom associated with this problem is the engine pinging. Engine pinging if you don’t know is a situation when you experience engine knocks. So, if the engine knocks more, it could be because of this problem. So, make sure you’re wary of engine knocking or a knocking sound under your car when accelerating.

And the last symptom on our list is the failed emissions test. Your car will not benefit from the poor work of the EGR valve. So, in most cases, you will fail the emissions testing if you live in a state where rules are extremely strict when it comes to these types of problems. Now let’s move on and see how we can diagnose the P0405 code.

How To Diagnose P0405 Code

Now let’s take a look at how to diagnose the P0405 code on your car. What you can do in order to tackle the problem?

Well, the first thing you would want to do is to check the wiring of your sensor. Make sure that there are no naked wires. If there are insulate them properly. Also, make sure that you inspect the connector for corrosion and that the contacts are good.

Once you do that, you can grab yourself a multimeter and test the wires that go from the PCM to the connector.

Tweak the multimeter to measure resistance, and put the key ignition in the ON position. And one of the wires which is the ground should create a beeping sound once you touch the connector. Red probe on the connector and black on the chassis.

The second wire that you want to test is the 5v supply line. Tweak the multimeter to measure volts and place the positive probe to the wire and the other probe to the chassis. The reading should be about 4.8 or close to 5v. If this is checked, your wiring is good.

And in order to test the sensor, you will have to backprobe the sensor. Start the engine first. Then put some metal clips from the rear of the connector to the signal wire and the ground wire.

Then place the red probe on the signal wire and the black probe on the ground. Anything below 0.6v is low voltage and the sensor is probably due to being replaced. A good sensor usually produces a voltage in the range of 0.6v and 3.3v as you can see in the video above. Now let’s conclude this article on the P0405 code.

P0405: In Conclusion…

In this article, we covered the P0405 code. First, we discussed the basics of car diagnostics and learned how you can diagnose a car. Then we elaborated more on the problem and learned about this EGR valve position sensor.

We covered the causes, symptoms, and diagnostics process that you can do with a multimeter and check if this sensor is working properly.

FAQs On The P0405 Code

Now let’s answer some frequently asked questions.

What Is An EGR Valve

EGR valve is the gas recirculation valve that is installed on diesel vehicles, as well as on some gasoline cars. This valve basically recirculates some of the exhaust fumes into the intake manifold and reduces NOx particles.

What Does EGR Stand For

EGR stands for Exhaust Gas Recirculation. This is a special valve that opens and closes to allow some of the exhaust gases to be redirected into the intake and burnt off. The EGR valve reduces NOx particles.

What Does EGR Delete Do

EGR delete is a simple block-off plate that is installed on the engine. The EGR is then removed and then the EGR is electronically deleted with software. If you don’t do this, you will still get the check engine light. We recommend avoiding doing these things and just replace the EGR with a new one.

What Does EGR Mean On A Code Reader

EGR valve is the Exhaust Gas Recirculation valve that is an emissions device. So, when you get a code concerning the EGR valve, it means that you have a problem with this component.

What Is The EGR Sensor

The EGR sensor basically determines the position of the EGR valve and how much open is this valve. Then the sensor reports to the PCM and the PCM has the data about the performance of the EGR valve.

How To Fix A P0405 Code

Well, the first thing you need to check is the signal wire and the connector, and make sure that these are in good condition. If everything is good with the wiring, you want to backprobe the sensor while the car is running. Install metal clips inside of the signal and ground wire and then connect the red probe to the signal and the black probe to the ground. Then start the car and tweak the multimeter to measure volts. A voltage for a good EGR should be at least 0.6v. If it’s lower, like 0.1v, then the EGR position sensor is bad and you have to replace it.

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