Home Diagnosis and TroubleshootingOBD Diagnostic Error Codes P0304 – How To Diagnose This Code?

P0304 – How To Diagnose This Code?

by Jordan Harris

Are you having troubles with misfires and you experience the P0304 fault code? Well, if that’s the case, then you are at the right place because there will be a lot to cover on this topic where we will learn a lot when it comes to this trouble code.

Trouble codes are a real pain when it comes to solving. It’s the same thing as you are solving a puzzle, it could get a really tricky thing to deal with. Especially this error code could be caused by more than a few things.

This is why you need to put up your sleeves and dive deeper into troubleshooting and understand how everything works. This is quite an essential thing to do because troubleshooting is a real mastery. And if you consider yourself to be a problem solver, then this might be the right task for you. But when it comes to this error code, you need to come well prepared. And we are going to prepare you well for that.

First, we are going to dive into the basics of car diagnostics and list the tools that you will probably need to diagnose a problem like this. Then we will learn the common causes of this problem as well as the symptoms that this P0304 code is producing.

Knowing these things will set you miles away from the competition. Then we will focus on the main thing which is diagnosing the problem as well as fixing it and the cost involved in this. Lastly, we will answer some common questions. So, if you want to learn more, follow along.

Introduction To Car Diagnostics

Now before we jump into more complex topics like the trouble code P0304 Ford, let’s take a look at some of the basics when it comes to car diagnostics. Knowing the basics is also helpful. You will have good input on how it is done and what tools you are going to need.

If you feel like you are well prepared, you can jump to one of the following chapters where we will cover the causes and symptoms. If not, keep up with us for a bit.

Modern cars are not like cars back in the day when you just opened up the hood and did small tweaks on the carburetor or a little bit of valve adjustment. They’re on a whole different level in comparison to these classic motors.

Modern cars are bloated with sensors and these sensors are used all over the car. There are a ton of them. And normally a ton more things fail down the alley. Whenever some sensor fails or a different problem occurs, you will probably be notified with a check engine light.

This check engine light can mean anything. And in order to diagnose the matter, you will need to get an OBD2 scanner tool. With this tool, you can get the right input if you are suffering from a P0304 Toyota code. If you do, then the next thing would be to learn more about this code and what does it represent. And in the following chapter, we are going to cover precisely that.

P0304 Cylinder 4 Misfire Detected

Now as we cleared what is car diagnostics and we determined that we have the P0304 trouble code, let’s learn more about what this code is all about. What this code represents?

The short message that you will get when you scan the car for codes is the message P0304 cylinder 4 misfire detected. But what does this means in the first place? What is a misfire?

Well, an engine misfire is a situation when the combustion inside of the cylinder is not as it should be. This means that the combustion does not occur whenever there is a need to.

It can occur prematurely, for example, if you are running a low octane fuel or you have another issue in the cylinder. Or it can get delayed in a different case.

Both of these scenarios are not good. Both of them mean that you have trouble with your car and cylinder 4 is not performing as it should.

If you continue driving like this, you might face some other problems down the alley. Mainly problems with spark plugs or other components that ignite the fuel.

When it comes to this problem, there are a lot of variables and a lot of different things that might be causing a misfire in this specific cylinder. But more on that later on, now let’s see if this problem is really serious or you can skip it and pretend like the issue with the P0304 code is not there.

Is This P0304 Code Serious?

We cleared what the error code P0304 Ford represents, now let’s learn if this problem is a serious issue. And the short answer is yes, it is serious. But not by a big margin.

As we noted, this problem with the cylinder 4 misfires could be caused by a lot of different things. So, you need to troubleshoot and troubleshoot day and night to find the right cause.

When it comes to the seriousness of the issue, we can say that you can drive like this for a while. But ignoring this problem, in the long run, can be really detrimental to your engine.

When there is a misfire inside of the cylinder, you know that there is something that doesn’t work as it should. So, you should not ignore it. Many people decide to only erase the codes and remove the check engine light. But you should not do this.

Whenever there is a check engine light, you can assume that this happens for a reason. And as we mentioned, there are a ton of reasons why the P0304 Ford code can occur. And in the following chapters, we are going to cover all of these causes.

Knowing the causes is going to help you when it comes to troubleshooting the matter and finding the right culprit. So, if you want to learn more, keep up with us.

P0304 Code Causes

Now as we cleared the basics of the P0304 Toyota code and learned what this error code is all about. Let’s now focus on sorting out the problem. Namely, learning the causes for this problem.

Everything happens for a cause and so is the case with the P0304 Toyota code. Knowing the causes will help you and show you where to look for the problem. Now let’s begin covering them in the following chapters.

1. Faulty Spark Plugs

The first and most probable cause for a P0304 Toyota code is a problem with the spark plugs. Spark plugs can foul over time and with age. And they can cause the car to misfire.

Specifically in this problem, this is an indication that you have a misfire on cylinder number 4. So, what you could do is to swap other spark plugs with the one on cylinder number 4 and see if this problem appears on the cylinder where you installed this spark plug.


If it does, then you know that this spark plug is bad. Replace it and you are good to go. If not, move on to the following causes for the P0304 Toyota code.

2. Bad Ignition Coils

The second most probable cause for a P0304 code is a problem with the ignition coils. The ignition coils could be bad and cause the cylinder to misfire. But how does this happen?

The coils are in simple words electrical transformers. They transform the power from the battery into a powerful electric current that powers up the spark plugs. Sometimes though, these coils are not working well and cause problems. They also cause error codes.


So, when you get a P0304 code you probably also have another code indicating a bad coil.

If not, you can swap the coils and install a different coil in this cylinder, this is how you determine a bad coil.

3. Bad Distributor Or Wires

In some older cars, the distributor or wires could cause a P0304 Honda code to appear. This is most common on older cars, the late 80s, and early 90s when engine coils weren’t a thing.

EFI cars were still using distributors and wires and they can fail as well. So, if you are running one of these systems on your car. You can try to experiment with the wires and see if a certain wire doesn’t deliver power.

But it’s highly likely that the distributor is bad and you will need to replace both the distributor and wires with new components if you want to sort out this problem. Now let’s move to the next cause.

4. Broken Injector

The next cause that we would like to elaborate on is the case with the broken injector. An injector can get clogged up. But as well it can stay open and leak fuel non-stop.

This could lead to misfires on a specific cylinder like in the case with the code P0304 Honda that we are covering in this article.

So, you can also do the same trick and swap out some of these injectors and determine which one is bad. This is one of the oldest tricks in the book, when you are dealing with the same components, just swap them out and see if the problem moves on to a different cylinder. If it does, then the problem for sure is in the injector. If not, move on to the next probable cause.

5. Vacuum Leak

The next thing that we would like to cover a cause for the P0304 Honda code is the situation with the vacuum leaks.

Modern cars can have a ton of vacuum leaks around the engine. These hoses are made out of rubber and this material deteriorates over time and can cause troubles like this for a lot of owners. Sorting the problem can be really tricky because you will have to use a smoke machine in the process if you want to find these leaks.

Common things that can cause vacuum leaks are the EVAP system, PCV valve, EGR valve, and a ton more components. So, whenever you have problems with one of these, you will also get other codes along with the P0304 code.

6. Bad Valve Timing

Another probable cause that could make a P0304 code is the bad valve timing. The valve timing in that specific cylinder can be off by a big margin and basically cause the engine to misfire.

The valves will not open and close when they have to and your engine will run rough. And it is a really good idea for you to check out these valves and see how they work. If there is noise coming from the top of the engine it means that there is a valve that doesn’t seal well. It could require a stem seal replacement or adjustment.

Later on, we will learn how you can adjust these valves while your car is running. Now let’s cover the other causes that are worth mentioning.

7. Other Causes

Now let’s list some other causes that are less likely to cause this code, but they still can cause the problem. The top cause for this problem could be a leak inside the cylinder. The piston rings could not seal well, or possibly the valves are not in the best shape, or the head gasket is bad. All this relates to low compression. So, when you have a problem like this, it is worth checking the compression that this cylinder produces.

Other causes can be a low octane fuel that could cause early detonations and misfires, a bad camshaft sensor, or low fuel pressure. Make sure that you address these things as well when it comes to the P0304 Honda code. Now let’s see the symptoms.

P0304 Error Code Symptoms

Now let’s cover the symptoms of the P0304 code. As we learned the causes, the symptoms are also needed to be known if you want to diagnose the issue.

The check engine light is understandably the reason you are here. This is why we will not going to include it in our list of symptoms. Now let’s begin with the symptoms.

1. Engine Misfires

Besides the check engine light on the dash which is an understandable occurrence in this situation, there will be a problem with engine misfires when you have a P0304 Toyota code. Since as you know, this code means that cylinder 4 is misfiring. But what does this mean?

Well, if a cylinder is misfiring, it means that there is a problem with the timing of the explosions inside the combustion chamber. They either occur too early or too late.

This type of work will cause the engine to run strangely and produce pops and bangs from the exhaust pipe. Now let’s move to the next probable symptom.

2. Engine Runs Rough At Idle

The next probable symptom with the P0304 code that you will experience is the situation when the engine runs rough at idle. The engine revs will fluctuate. They will increase and decrease and cause a lot of trouble.

There will be vibrations as a consequence of this type of work. You will notice how the engine will be shakier than usual and also noisier.

3. Low Engine Power And Poor Acceleration

The third symptom that you will probably notice when you have a P0304 code on your vehicle is low engine power.

If one of your cylinders is misfiring means that it doesn’t produce enough power to maintain proper engine performance.

So, your engine work will suffer and the car will become slow and will lack some response when you press on the gas pedal. A situation that you probably don’t want.

If this is an engine that is relatively weak and has low displacement, you will notice this power loss big time. This is why you need to diagnose the P0304 code. But more on that in a bit.

4. Fuel Smell

And the last symptom when it comes to the P0304 code that we would like to cover is the smell of gas that will be quite noticeable when you start to experience a problem like this.

This can be caused by a faulty coil or a faulty spark plug. The fuel will not burn in the cylinder and will be expelled from the exhaust. Thus producing a strong odor of gas.

But this could also hurt other components in your car. Namely, the catalytic converter and the O2 sensor. And these are really expensive components that you don’t want to replace because of a faulty spark plug. This is why diagnosing the problem sooner should be your top priority. And that’s what we are going to cover up next.

How To Diagnose & Fix A P0304 Code?

Now as we covered the causes and symptoms of the P0304 Chevy code, let’s now learn more about how to diagnose the matter. How you can tell what is causing this misfire in your car?

Well, for this you will need to improve your troubleshooting skills. Meaning that you will need to use the knowledge that we have shared and diagnose the problem.

If you have this code, then you probably would have other codes if a coil or an injector is bad since these components are connected to the ECM. If you don’t have, you can try to swap out components to determine.

Start with the spark plugs, and if the spark plugs are good, then move to the coils and then to the injectors, and so on. If all these components are in good condition, you will need to dive deeper.

You will need to see if the timing is good on your car, check for noises coming from the valves and remove the cover and try to tighten the valve a bit with a key.

Also, check for a blown head gasket, compression in the cylinder as well as fuel delivery-related issues. Everything needs to be checked if you want to diagnose the problem successfully.

Once you diagnose the issue, just replace the component with a new one and you will be good to go simple as that. If there is a compression-related problem you might be needing some machine work on your car if you want to resume driving and sorting the P0304 Chevy code.

Cost To Fix The P0304

Now as we covered how you can diagnose a P0304 Honda problem, now let’s learn more about the cost to fix this issue. How much can you expect to pay for a problem like this?

Well, if there is a simple problem. Namely, spark plug, you can get away with $10 to replace one single plug or about $120 to replace them all. If the problem is a coil, you can expect to pay about $150 for a new coil.


Injectors are more expensive, especially if the engine is direct injection. You can expect to pay about $300 for one of these.

If the problem is compression-related, then there will probably be a need for a little bit of machine work on the valves to get them working right and this could be a bit expensive, costing about $1,500.


In this article we have covered a lot when it comes to the P0304 Honda code, we learned what is diagnostics and what you will need to pull this off. You will highly likely be needing of an OBD2 scanner tool to get this code.

Then we learned the causes as well as the symptoms that this code is associated with. Namely, the code is frequently caused by bad spark plugs or ignition coils.

Lastly, we covered how you can diagnose the code P0304 Dodge and how to fix it. Also at what cost this will be possible.


Now let’s answer some frequently asked questions when it comes to this problem.

How Do I Fix Code P304?

Fixing this code can be tricky. You will need to start troubleshooting and swapping out different components to determine which of them is faulty. And this problem frankly gives you that chance in order to sort this issue quickly. You need to be a bit persistent and develop your troubleshooting logic.

What Does Code P0304 Mean

This is a trouble code that represents a problem. It indicates that you have a misfire on cylinder number 4. This problem might be attributed to a bad spark plug, bad ignition coil, bad fuel injector, or problems with the valves not being perfectly timed out.

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