P0351 – Ignition Coil A Primary-Secondary Circuit Malfunction Fix

by Conner Mckay

Have you started experiencing some strange symptoms like the poor ability to start the engine and engine misfires? And you diagnosed the code P0351? Well, if this is the case, then you are at the right place because, in this article, there will be quite a bit to cover on this topic.

When it comes to problems with your car, doing good and thorough research is always a good idea because you want to sort out problems by yourself. And to avoid paying a ton of money on mechanics for fixing simple things such as this. Even something simple like how much is for a car diagnosis tends to be costly.

And honestly, when it comes to the check engine light and the DTC codes, you should always come prepared to tackle the problem. You just need to get the right tools needed for this type of work. And by this, we mean an OBD2 scanner and a multimeter.

But don’t worry even if you don’t have one of these tools, in the following chapter we are going to elaborate on everything about them. First, we are going to learn what is car diagnostics and the tools needed for this type of work.

Then, we will cover the ignition coil and the location. Once we clear that out of our way, we will discuss the P0351 code, causes, symptoms, and how to diagnose the problem. So, without further ado, let’s start discussing.

Car Diagnostics

One topic that we would like to cover before we dive into the P0351 code is understanding what is car diagnostics in general. This is a topic that confuses a lot of people. So, we feel the need to further elaborate on it and understand more about how this process is done the right way.

As you probably know, modern cars are all packed with technology. There are dozens of sensors that communicate with the brain of the car. Which is the Powertrain Control Module, also known as the PCM, ECM, or ECU.

But what is worth noting is that not only sensors are connected to this computer. But also some of the ignition components are also regulated by the computer.

Which in our case are the ignition coils, injectors, and fuel pump. This is why we always have the best fuel economy and the best fuel efficiency possible.

Modern cars are basically night and day difference from their old counterparts in performance and efficiency. And you should not get frustrated that you got the check engine light.

In fact, this is a good thing since you know that you have a problem. And in order to find out what is causing it you only need an OBD2 scanner tool to learn more about the codes. Like in this case with the code P0351.

And then, with the help of a multimeter, you can inspect the component and see if it works well. And there are also some other tricks that we are going to learn later on. But what is the ignition coil? More about that, next.

Ignition Coil

Now before we dive into the P0351 code, let’s first learn more about what is ignition coil in general and understand a bit more about the general concepts of ignition coils and what they do.

The ignition coil is also known as the induction coil. This is basically a laminated iron core that is surrounded by two copper wires. And this ignition coil has an open magnetic circuit that creates a magnetic field. And this energy which is stored in this magnetic field is sent directly to the spark plug.


Now let’s learn what a coil is in layman’s words. The ignition coil is a simple electric transformer. As you probably know, you have a 12v supply from the battery. But this power is not enough to create an ignition.

So, these ignition coils are needed in order to transform the 12v into a high-voltage current which is needed to create a spark.

This is why we can say that ignition coils are pretty crucial when it comes to the ability of the car to start. Without them, it would be impossible for the engine to start. But where they are mounted? Let’s learn more about that next before we explain the P0351 code.

Ignition Coil Location

Another thing we would like to elaborate more before we dive into the definition of the P0351 code is the ignition coil location. Where are these ignition coils located?

What is worth noting is that back in the day, there was only one ignition coil that was then sent current to the distributor (unless you’re experiencing the symptoms of a bad distributor). Then this power was sent from the distributor to each wire for each of the cylinders.

But nowadays, this system was abandoned and there is far less wiring inside the engine bay. For this purpose, carmakers created individual coils for each cylinder.

So, in modern cars for each cylinder, there is a coil that is plugged into the spark plug and then connected to the wiring harness.

So, if you have an inline-4 engine, you have 4 coils, if you have an inline-6 or a V6 engine you have 6 of these coils. And you can have up to 12 coils if you run a V12 engine. And this means more things to break. That’s why reliability often suffers on these engines with a lot of cylinders.

But what about the P0351 code? More about the code, we will elaborate next.

Ignition Coil A Primary Secondary Circuit Malfunction

Now that we learned more about the ignition coils and how they work. Let’s now take a look at the definition of this code on your car.

The simple definition of this code is “P0351 Ignition Coil A Primary Secondary Circuit Malfunction”. But what does all this means in the first place?

Well, this is a code that is triggered when there is a malfunction in the ignition coil A. If you don’t know, ignition coil A is the coil that is found on the first cylinder. So, in order to find this coil you have to look for the firing order for your engine.

Even though it is not that difficult to find, if you own an FWD car, this coil is far left when you open the hood. Or if you have an RWD car with a V-engine, this coil is located on the right side.

When it comes to the code, we can say that there is a problem with the primary or secondary circuit of the ignition coil. So, when the PCM requires specific signals from the ignition coil and these signals are not up to the recommended spec, it will decide to trigger the P0351 code.

Even though this does not mean that the ignition coil is faulty. There could be a few causes behind it. And in the following chapters, we are going to elaborate on them.

Causes For The P0351 Code

So, we have covered the P0351 code definition and we learned that this is caused by a problem with the ignition coil A which is located on cylinder number 1.

Now let’s take a look at the causes of the P0351 code. Knowing the causes will help you determine what could be the root of this problem. So, let’s dive in.

P0351 Causes #1: Fouled Spark Plug

Now let’s begin with the causes for the P0351 code and learn more about what could cause this problem.

First on our list of causes for this issue is the spark plugs. Spark plugs can wear out and start to misfire. The electrodes can get damaged and when they get damaged, the coil will definitely have a hard time performing well.

If so, it might be handy to learn about what does a bad spark plug look like, as well as how to check your spark plugs. Not to mention, understanding the symptoms of a bad spark plug.

spark plugs

So, in this case, a code can be triggered such as this. And this type of work with a fouled spark plug can cause the coil to fail. So, this is why it is really essential to check the spark plugs first (and know how to read spark plugs) before you jump to conclusions and replace the coil itself.

More on that, we are going to learn later on. Now let’s continue with the causes of the P0351 code.

P0351 Causes #2: Bad Spark Plug Wires

The next cause for this problem with the P0351 code that we are going to elaborate on is the problem with the wires.

If you are running an older engine from the late 90s that has EFI ignition, you probably have some thick wires that go from the distributor to the spark plug.

This is an old-school design that is no longer present on newer cars. But what can happen is that these spark plugs can fail and also cause a problem like in our case with the P0351 code.

So, it is definitely a good idea to check the wires accordingly and replace them if necessary.

P0351 Causes #3: Bad Ignition Coil

The next cause is probably the most common among the symptoms of a bad ignition coil concerning the P0351 code. And this is a defect in the ignition coil itself.

Ignition coils tend to fail, some more often, some of them to a lesser extent. For example, if you have a Ford, this is a completely normal thing since these coils fail even if you look at them the wrong way.

Also, VW coils fail very often as well. Overall, after the spark plug, a bad coil is the second lead to this problem with the code P0351.

So, this is why you will have to test these coils and see if coil A is broken or not. Now let’s move on to the next cause of this problem.

P0351 Causes #4: Poor Connector

The next cause for this problem with the P0351 code is often the poor connection between the coil and the wiring. Sometimes coils can get unplugged since there are a lot of vibrations inside the engine bay.

Or the connector can crack over a long time of usage and then these cracks will allow some intermittent signal to be transferred and trigger a code like this.

So, unplugging the connector and checking out the condition of the coil is always a smart thing to do since it will not cost you anything.

Also, it is often advised to spray it with some electrical contact cleaner. This will make sure that the contact is good between the harness and the coil itself. Now let’s move on to the next cause for the P0351 code.

P0351 Causes #5: Bad Coil Wiring

The next probable cause for the P0351 code on our list is the bad wiring that goes from the wiring harness to the ignition coil.

What can happen is that sometimes wires can melt and when wires melt, this can trigger problematic codes like in our case.

And not only that, but the ignition coil can also malfunction on the inside and not deliver the proper electrical current for the spark plug. So, proper inspection of the wires is also very much recommended practice in these cases. To check just in case, if something was rubbing against and causing malfunctions.

P0351 Causes #6: Vacuum Leaks

Now let’s move on to the next cause for the P0351 code and this is the vacuum leaks. So, what are vacuum leaks?

Well, a vacuum leak is any leak that allows unmetered air to enter the intake. So, for example, a cracked intake manifold or a bad manifold gasket can cause a vacuum leak that can trigger this code.

As well as a bad EGR valve in some cases. We listed the EGR in this chapter, since this is a component that also helps with the help of a vacuum and if it fails it will cause similar issues. Therefore, don’t skimp on the vacuum leak repair cost to get your car running again.

P0351 Causes #7: Bad PCM/ECM

And the last on our list of causes for the P0351 code is the bad PCM. So, what is a PCM? PCM is the powertrain control module.

In general, this is the main computer of your vehicle that regulates everything. And sometimes this computer can also malfunction and trigger codes such as this.

This is the last resort. But if you notice that the code is there for no particular reason, then you should definitely check the PCM and see if it’s working properly. You could also diagnose other codes such as the P0603 code or the P2509 code. Now let’s move on to the symptoms of the P0351 code.

Coil Pack Symptoms

Now let’s cover the symptoms of a failing coil pack and see more about what they are telling us. How we can determine if we have a bad coil pack? Let’s elaborate in the following chapters.

1. Check Engine Light Is On

The first symptom of a bad ignition coil is the check engine light. The check engine light will be present if you have a bad ignition coil.


And codes like P0351 will be present. Or another code like P0352, P0352, P0353, P0354, and so on, depending on which coil is affected by this problem.

Now that we have the code P0351, this means that coil A located on cylinder number 1 is affected.

2. Car Difficult To Start

The second symptom on our list is the situation when the car is difficult to start. Starting a car with bad coils can be a real pain if you are asking me.

One of the cylinders will not like to work. So, if you have a smaller inline-4 engine, this situation will be very pronounced and noticeable. So, the car will simply struggle until it eventually starts. And even then, other symptoms like the ones we are going to cover next will be present.

3. Rough Idle

Another very common symptom of a bad coil and the code P0351 is the situation when you have a rough engine idle.

The engine will run really rough on idle and you will notice how the car simply runs on three cylinders. This rough idle will also cause vibrations inside the cabin and the car will start to shake a little bit along the way. A bad ignition coil is one of the most common causes of a rough idle.

4. Engine Misfires

Next on our list of symptoms of the P0351 code is the situation when you have engine misfires. So, what are engine misfires?


Well, engine misfires are uneven explosions that occur inside the cylinders. They often resemble small ticks or taps that come from the top of the engine. Usually from the cylinder that is mostly affected. From the exhaust, there will be also some pops and bangs and uneven gas flow.

Elsewhere, you could also use your OBD diagnostics scanner tool to diagnose if your car has a misfire. You’ll note misfire-related OBD error codes such as:

5. Poor Engine Performance

And the last on our list of symptoms when you have a P0351 code and a bad ignition coil is the problem with poor engine performance.

And the smaller the engine, the more noticeable this would be. So, when you put your foot on the throttle the car would feel really sluggish and slow to respond.

This is the case because there is no good combustion inside your engine. So, if you have a 4-cylinder engine that is making 120hp, without an ignition coil function properly, this engine would create only 90hp.

And the difference will be very noticeable. But how you can diagnose the P0351 code? Let’s elaborate more about that in the next chapter.

How To Diagnose A P0351 Code

Now let’s take a look at how you can tackle the P0351 code and learn more about how you can diagnose the problem on your car.

The easiest trick in the book that you can try when it comes to tackling a problem like this is to swap out the coils.

You know that cylinder number 1 is affected, so what you do in this case is swap the coils. Swap cylinder number 1 with cylinder number 2.

If the code moves to the other cylinder and now you get a P0352 code, you know that the coil is faulty and you have to replace it.

If the code stays on cylinder number 1, then you probably have a bad spark plug. So, you see how it is really easy to diagnose a problem like this.

You can also test the coil with a multimeter, you just tweak the multimeter to measure ohms and you place one probe on terminal number 1 and the other on terminal 2. If the resistance is good and the coils make about 6.5 ohms, this means that the coil is good. How you can test this, you can check in the video above.

Or, for written references, check out our guide on how to test a coil pack without a multimeter.

Ignition Coil Replacement Cost

Now that we learned how you can diagnose a bad ignition coil and the code P0351. Now we can move on and learn more about the total ignition coil replacement cost. How much is an ignition coil?

An ignition coil has an average price of $150. For a single piece. But prices can also go up to $350 for some more expensive models. This is why you should make sure to get the right coil for your car.

P0351: In Conclusion…

In this article, we have covered quite a bit when it comes to the P0351 code. First, we learned what car diagnostics is and how it is done. Then, we learned what is an ignition coil and also how ignition coils work.

After that, we covered the P0351 code and learned the meaning, causes, symptoms, and also how to diagnose this code on your car. Lastly, we elaborated on the cost of a new ignition coil (be wary of the differences between the ignition coil vs spark plug).

Frequently Asked Questions

Now let’s answer some frequently asked questions.

What Does PCM Stand For Car

PCM is an abbreviation that stands for Powertrain Control Module. In general, this is the main computer of the car that regulates everything concerning the powertrain with the engine plus the transmission.

What Is A Ignition Coil

An ignition coil is a component that has the ability to transform electric current. The ignition coil is a small transformer that transforms the low voltage from the battery into a high voltage current and then this current is sent to the spark plug and then the spark ignites the fuel.

Where Is The Ignition Coil

On older cars that have only one coil, the coil is mounted somewhere around the firewall. But on modern cars that implement individual coils for each cylinder, the coil is mounted on top of the spark plug hole.

Will A Bad Coil Pack Throw A Code

Yes, it would definitely throw a code. Mainly because these coils are connected to the computer and if there is a problem, the computer will alert you. What is good is that with a code reader, you can check which coil specifically is affected.

How To Test A Coil Pack With Multimeter

You can test the ignition coil for resistance. Just tweak the multimeter to measure ohms. Then place one probe in the 1st terminal and the other probe in the 2nd terminal. A good reading is about 6.5 ohms. But this measurement mainly depends on the type of coil that is used.

How To Test Ignition Coil Without Multimeter

You can just swap out the coils. If cylinder 1 is affected, swap the coil with cylinder number 2. If the code appears on the second cylinder, then this means that the coil is faulty.

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