Home Diagnosis and TroubleshootingOBD Diagnostic Error Codes P0303 – A Simple Misfire Or A Lot More Besides?

P0303 – A Simple Misfire Or A Lot More Besides?

by Jordan Harris

Are you experiencing a check engine light and the code P0303? If this is the case and you are in this situation, then you are at the right place because there will be a lot to cover when it comes to this error code and the ways how you can overcome it.

Having a problem like this can sometimes be a really frustrating thing to deal with because this error code can mean a lot of things and can be caused by a lot of things as well. It is not that easy to diagnose and fix. This is why you need to improve your knowledge when it comes to this type of error and understand what it means and what are the causes and symptoms of it.

Knowing this stuff will help you diagnose the matter much simpler with only common tools and basic knowledge about the matter. When you will understand how everything is connected and works as one, you will be able to understand the consequences of each component failing or running poorly. Which in this case is a misfire in cylinder number 3 of your engine. But more on that later in the article.

First, we are going to cover the basics of car diagnostics and learn how it’s done. Then we will cover the problem with the P303 code and learn more about it. After we will cover the causes as well as the symptoms of this code. Then we will learn how to diagnose and fix the issue quickly and effectively. So, if you want to learn more about solving this problem, follow along.

Understanding Car Diagnostics

Now before we elaborate more on the problem with the error code P303, let’s first learn more about car diagnostics in general terms. Knowing this stuff will be extra essential if you want to tackle this problem quickly and effectively. If not, you can move on to the causes of this problem as well as the symptoms that are very common to this issue. If not, keep with us for a bit.

Modern car diagnostics is not like back in the day when you only needed to adjust the carburetor or replace the spark plugs and wires and call it a day. Modern cars work with something known as ECM or PCM.

This ECM or PCM as some carmakers would call it is the computer, the brain of the car also known as an engine control module. This module communicates with all of the different sensors that are mounted on the car. And frankly, there are dozens of them all over the car. Meaning that you have a lot of troubleshooting to do when you have a problem with your car.

Each of these sensors when fails it starts to develop trouble codes also known as DTCs. These codes can only be checked if you plug in an OBD2 scanner tool into your car and get good input on them.

Then based on the input, you can move on to locating the problem. In most cases, a lot of these problems are sensor-related. While some can be related to problems with valve adjustment and other possible malfunctions inside of the engine.

Now as we cleared out everything, let’s move to the main focus of this article and that is the P0303 Nissan code. Let’s learn what it means first.

P0303 08

Now that we know what car diagnostics are, let’s now focus on our main problem. And that is the P0303 code. What this code means and how serious this problem really is for you as a car owner?

This code basically means P303 cylinder 3 misfires detected. Meaning that a misfire on the number 3 cylinder is occurring and the engine runs rough.

When a car misfires, you will be able to tell it straight away. There will be ticks coming from the valve cover and the engine will not be happy at all. In addition to this, there will be noises coming from the rear of the car. The exhaust will pop and bang. The flow of gas will not be equally distributed. But what is this misfire?

A misfire is simply bad ignition timing. The explosions inside the cylinder are not properly timed. So, when you have a misfire the explosions happen prematurely or are being delayed. This is not good for your engine because the engine always likes to run equally on all of the cylinders.

This is why you need to solve the problem as soon as possible and make sure that the problem never repeats again. Sorting this problem can be really tricky because there could be a lot of things that could cause it. But we are going to help you out when it comes to diagnosing the P0303 Ford code and deliver you the best answers possible.

How Serious Is The P0303 Code?

Although some would say that this code P0303 is a serious issue, it will not cause you not to be able to drive the car. Since only cylinder number 3 is misfiring. That’s one cylinder out of 4 if you are running a straight-4 engine.

Meaning that you will still be able to drive the car. But you will not be able to get the most out of it since this affected cylinder will not deliver power or will deliver slightly less power depending on the seriousness of the misfire.

That’s why you should not be gravely concerned that your car will simply stop working and cause the engine to stall in the middle of the road.

What is important though is that you scan the car for codes and be aware of them. Then sort out the problem whenever you can. But what could be the causes for the P0303 Honda? Let’s see more on that in the next chapter.

Causes For A P0303 Code

Now as we cleared the basics when it comes to car diagnostics and we learned what the P0303 Ford error code really is. Now let’s cover more on the causes of this problem happening.

As you probably know, there is always a cause for everything that happens and so is the case with this error code. So, what are the causes for this issue? Let’s find out.

1. Spark Plugs

The first thing that you want to look at when you are having a problem like the P0303 code is the spark plugs. More precisely, the spark plug on cylinder number 3.

You will have to remove the spark plug on this cylinder and inspect it. Make sure that the spark plug is in good condition. If it’s not looking good and you can notice some damage to the electrode caused by oil, or gasoline, then you can go ahead and replace it with a new one.


It is advised for you to replace all of the spark plugs that are on your car. This is how you are going to be sure that the problem doesn’t repeat itself anytime soon.

2. Ignition Coils

The second most probable cause for the P0303 Toyota is a problem with the ignition coils. The ignition coils can fail and cause a lot of trouble for you as a car owner. These coils sit on top of each cylinder and basically work as electrical transformers.

They transform the low-power electricity into a high-power current that powers the spark plugs and starts the engine. If one of these coils is bad, you will have problems such as misfires.


Here is a simple troubleshooting tip when you are dealing with coils. Swap the coil with another coil from a different cylinder. If cylinder number 3 doesn’t misfire anymore then the problem was in the coil. If this doesn’t solve your problem then you should look for the solution for the P0303 Dodge elsewhere.

3. Bad Distributor And Wires

In some older cars that use electronic ignition, there could be still found some old-fashioned distributors and wires. These are mostly cars from the 90’s or later 80s era.

The distributor and wires were the old way of using engine coils. Meaning that the distributor could fail and cause problems like this with the P0303 code.

But the bigger problem was the wires that often failed. If you run a car like this, it is advisable for you to check these components and see if they are working right. Swap out some of the wires and see if the problem is repeating itself on cylinder number 3. If it is, then the distributor is probably the culprit for this issue.

4. Bad Fuel Injector

A bad fuel injector can also cause the P0303 code. This is especially notable for a leaky injector that leaks a lot of fuel whenever there is not necessary to do so.

Meaning that whenever you have leaks, the fuel could get ignited sooner than previously anticipated and the misfire will be inevitable. It is a clear sign that there is something wrong with this specific cylinder number 3 and the code is probably caused by the injector itself.

Your best bet would be to swap injectors from other cylinders and see if this solves your problem. If the injector is faulty, replace it with a new one.

5. Bad Valve Timing

Another very probable cause for the P0303 BMW is the problem with bad valve timing. But what does this mean?

Well, this means that the valves do not open and close whenever they are needed to do so. Thus, creating misfires inside the engine. When this happens on one cylinder, only the affected cylinder will produce misfires on your car.

In this situation, the valve cover has to be removed and the valves need to be adjusted with a special tool for adjusting valves. Some people remove the valve covers and start the engine. Then they tighten the valves until there are no more misfires heard. A great idea that you can try to adjust your valves. Now let’s move to the next probability.

6. Vacuum Leaks

The next cause for the P0303 code that we want to discuss is the vacuum leaks that often can happen and cause a situation like this.

And honestly, there are a ton of vacuum lines across the car. You got an EVAP system, EGR valves on some cars, PCV valve. All these things can malfunction and make the car misfire and develop issues like this.

So, if everything is working up to spec, then you would want to check these things as well and make sure that you don’t have any vacuum leaks that can lead to a misfire on your engine.

7. Other Causes

Now for last, let’s list some other less probable causes for the P0303 Honda code to appear. These causes include low fuel pressure caused by a bad fuel pump, bad camshaft and crankshaft position sensor, poor quality gas, loss of compression in cylinder number 3. Overall, these are the things that can cause this code from happening. Now let’s learn the symptoms.

Symptoms Of A P0303 Error Code

Now as we went through the causes for this problem with the code P0303 Ford. Let’s now take a look at the symptoms that you are going to experience when this code is present.

Knowing the symptoms is also useful when it comes to diagnosing and making assumptions about what could be the problem behind this error code. So, let’s begin.

1. Check Engine Light

As you would probably assume when it comes to the code P0303, the first symptom that you will notice will be the check engine light.

And without properly diagnosing this code you will not even be able to tell if you have a P0303 Ford code or a P0303 Nissan code. Depending much on the make and model.


This is why an OBD2 scanner is a tool that you want to get and make sure that you always diagnose any issue with your car as quickly as possible with minimal effort. So if there is a check engine light, move on to diagnosing it as soon as possible.

2. Engine Misfiring

The second thing that will be most commonly associated with this problem of the P0303 code as you would assume is the situation with the engine misfiring.

This will be extremely noticeable if you run an engine that is low on cylinders. If one of them is misfiring, then deterioration of performance will be quite noticeable for you as a car owner. This is why you need to look at the causes that we explained before in the previous chapters and based on these probabilities determine what is causing this P0303 cylinder misfire detected.

3. Engine Runs Rough At Idle

The third symptom in our list of symptoms concerning the P0303 code will be the rough engine idle. The engine will not be happy at all when it is misfiring and will show this to you.

You will start noticing how the RPMs will fluctuate and the engine will not run perfectly smooth. This is basically a dead giveaway that your engine is misfiring and this needs to be checked as soon as possible if you want to avoid any bigger problems with other components that are affected by this issue.

4. Poor Engine Performance

Poor engine performance will also be quite a common occurrence when you have a P0303 code. This code as we explained is causing the engine to misfire.

Since the engine misfires, there will be a considerable loss of power from the engine. Especially if you are running a small displacement engine where the poor engine work from a single cylinder can be felt considerably on the performance of the vehicle.

You will highly likely face hesitation and the car will feel sluggish and slow to respond. This is why when you notice a loss of performance from the engine. It is of utmost priority to address the problem as soon as possible.

5. Smell Of Gas From The Exhaust

And the last symptom that we are going to cover will be the smell of gasoline coming from the exhaust. If there are misfires there will be some unburnt fuel that will escape from the exhaust and you will highly likely be able to smell this gasoline inside of the cabin. Whenever you do so, move on to diagnosing the matter as quickly as possible. And that’s what we are going to cover up next.

How To Diagnose & Fix P0303?

Now let’s jump into diagnosing the problem of the P0303 BMW code. Let’s learn how you can diagnose and quickly uncover this issue.

We can assume that you already have done the diagnostics with an OBD2 scanner since you are aware that you have the code. Now let’s see what are the possibilities for you when it comes to troubleshooting and experimenting to determine quickly what is causing the engine to misfire on cylinder number 3.

The first thing you want to do is to swap spark plugs from other cylinders to cylinder number 3. See if the problem persists with a new spark plug. If it is, move on to the coils.

Swap out the coils and see if this code persists. If it does, then move on to the injectors and swap an injector from another cylinder to see if the code persists on number 3. 99% of the chances are that you have a problem with one of these three components.

If else, the timing might be off. In this case, you will have to do a valve adjustment. Remove the valve cover and start the engine. Tighten the cylinder number 3 valve with a key until the sound disappears. This will hopefully solve your problem permanently.

If else, you are highly likely are having a problem with some other sensor like the camshaft position sensor. This is why you need to make sure that you see if there are any other codes along with the P0303 code.

If there is not, it might be worth checking if you lose compression and if the valves are sealing well on this specific cylinder. A Head gasket check is also mandatory to determine that you have a proper seal.

Cost To Fix The P0303 Code

Now as we covered the problem with the P0303 trouble code, we learned what are the components that could be causing it and the symptoms that you will experience along with this problem. Let’s now take a look at the costs involved in sorting this problem out.

If the problem is caused by a spark plug, then you will highly likely spend the least amount of money to sort this problem out. A spark plug is less than $10.

Ignition coils on the other hand are more expensive and could cost you about $150 to $250 per coil. Which is not cheap.

Car problems troubleshooting repair OBD codes diagnose

Fuel injectors are also expensive if you are running a direct injection motor. These can cost up to $500 to be replaced. Regular ones are about $100. Labor for all these parts will also cost about $150.

If the problem is mechanical and there is a problem with compression and bad valves, then this problem can get expensive. You can pay up to $1,500 to sort these problems out.


In this article, we have covered the problem with the P0303 code and we learned that this is a DCT code indicating a misfire on cylinder number 3.

Then we have covered the causes of this problem which include bad spark plugs and coils, bad injectors, and other components. After, we learned the symptoms that this problem produces.

Then we covered how you can diagnose and sort the issue. Also at what cost will all be able to be done.


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

How Do I Fix Code P0303

Fixing this code really depends much on the root cause of the problem. Whether it is a bad valve timing, bad spark plug, ignition coil, or possibly a problem with the compression. The matter has to be diagnosed the right way in order for the issue to be uncovered quickly.

What Does Code P0303 Mean

This code is a trouble code that means that there is a misfire on cylinder number 3. Meaning the combustion in this cylinder is not like in the other cylinders and this could lead to performance loss and other performance-related issues on the car.

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