How to Fix P0303 Code: Solution for Cylinder 3 Misfire

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an OBD scanner with P0303 code on the screen

When the Check Engine Light illuminates on the car dashboard, car owners often dread what is wrong with their vehicle. It could signify a significant problem or something as simple as a loose gas cap. However, specific error codes are more severe than others, and one of these is the P0303 trouble code.

Is this the one your OBD scanner is showing? Don’t worry! This guide will list the symptoms, causes, and possible fixes for this error code. Let’s get started!

What is Code P0303?

The P0303 is a diagnostic trouble code for a cylinder misfire on cylinder 3. A misfire is a condition in which there isn’t enough combustion on one or more cylinders. It usually happens when the timing of the ignition is off, or the engine is not firing correctly on one of its cylinders.

Most engines have four, six, or eight cylinders. More cylinders mean more power to make your vehicle move. When cylinders misfire, the engine will still run but with less power and less efficiency.

Symptoms of Code P0303

Before your vehicle throws a P0303 code, you will usually notice other symptoms first. The most common symptom with this trouble code is the Check Engine Light. Aside from it, you may also see:

  • Reduced fuel economy
  • Loss of power while accelerating
  • The vehicle may shake or vibrate when idling.
  • The engine will rumble distinctly
  • Inconsistent Revolutions per Minute (RPM)
  • Rough drive while moving

Causes of Code P0303

A few different things can cause cylinder misfiring. Here are the most common causes:

  • A vacuum leak
  • A clogged fuel injector
  • Intake manifold leaks
  • Failed ignition coil pack
  • Defective spark plug
  • Faulty manifold gasket
  • Damaged spark plug wires
  • Problem with the vehicle's PCM module (uncommon)

How to Diagnose Error Code P0303?

As we've always said, when diagnosing a trouble code, you must always know what the causes are to test for them properly. However, in the case of P0303, the causes above seem overwhelming, right?

Looking at those might lead you to think that this will be a complicated code to diagnose, but it's not. Well, no matter how easy or complicated the process is, we will help you figure out where to start!

Check for Other Diagnostic Trouble Codes

The first step in diagnosing any trouble code is to make sure you scan for other error codes. If they are present, they might indicate other problems or the real root cause of your car's problem. If other codes are retrieved, write them down and solve them before the P0303 code.

Perform Visual Inspection

The second step and one of the most important steps is to perform a visual inspection. This will help you rule out any apparent problems causing P0303. Check for any loose, damaged, or burnt spark plugs.

Also, inspect the spark plug wires and look for any cracks, fraying, or damage. Don't forget to look at the ignition coil and see if it is damaged in any way. If any damaged wires are found, repair them or replace them if necessary.

Check for Vacuum Leaks

One of the most common causes of P0303 is a vacuum leak. The engine needs a certain air/fuel mixture to run correctly. If there is a leak, it can cause the engine to run lean.

Use a carburetor cleaner to check for a vacuum leak and spray it around the intake manifold gasket. If you have found a leak, you need to replace the intake manifold gasket.

Check for Fuel Injector Issues

Another common cause of P0303 is a clogged or damaged fuel injector. This can be caused by several things such as deposits, corrosion, or electrical issues. To check the fuel injectors, remove them and have them tested.

You can also check for fuel injector issues by unplugging each one, one at a time. If the misfire goes away, you've found your problem. You'll need to either clean or replace the fuel injectors to fix it.

Check the Ignition System

Another common cause of P0303 is an ignition system problem. This can be caused by several things such as spark plugs, spark plug wires, or a coil pack. To check the ignition system, follow these steps.

  1. Before you begin, make sure to wear safety goggles or any eye protection. Also, make sure to have                   insulated tools and pliers to protect you from electric shock.
  2. Locate the ignition coil by opening the hood. Though its position varies from car to car, it is usually near          the fender or fastened to the distributor's bracket.
  3. Once the ignition coil is found, remove the spark plug wire from the terminal.
  4. Use a spark plug tester, and attach the spark plug tester to the wire. Place the alligator clip on the ground.
  5. Now crank the engine, and if you see a spark jumping from the tester to the ground, the ignition coil is            working fine. If not, then it needs to be replaced.

How to fix Code P0303

After thoroughly diagnosing all the possible causes of a P0303 code and finding out the main reason, you may proceed to fix the P0303 code. However, you must be mechanically inclined to fix this problem, as it involves working with tools.

If you are not car-savvy, then you may take your vehicle to a nearby mechanic or dealership, and they will fix the P0303 code for you. But if you believe you can do it yourself, here are the steps involved in fixing a P0303 code:

Step-by-Step Guide to Replacing the Ignition Coil

If you find out that a faulty ignition coil is the cause of the P0303 code, you should replace it. To perform it:

  1. Locate the defective ignition coil. It should be easy to find as it will be the one that is not sparking.
  2. Unplug the electrical connector of the ignition coil.
  3. Remove any bolts or screws holding the coil in place, and then remove the coil.
  4. Get a replacement coil and install it in the reverse order of removal.
  5. Clear the P0303 code from your computer and take your car for a test drive to see if the code returns.

Step-by-Step Guide to Replacing the Spark Plug

If you determine that a spark plug is the cause of P0303, you will need to replace it. To do it:

  1. Locate the faulty spark plug. It should be easy to find as it will be the one that is black and soiled.
  2. Unscrew the spark plug with a ratchet or socket.
  3. Pull out the old spark plug.
  4. Install the new spark plug by screwing it in by hand until tight.
  5. Use the ratchet or socket to tighten the spark plug further. Do not over-tighten it as this can damage the       threads.
  6. Repeat this process for each spark plug that needs to be replaced.

If replacing the ignition coil and spark plug didn't work, you can also try and replace the PCM (Powertrain Control Module); however, it is uncommon. In most cases, returning the spark plug would clear the code P0303.

Common Mistakes When Diagnosing the Code P0303

Because many possible causes are associated with this error code, it is common for mechanics to misdiagnose P0303. Some of the most frequently made mistakes include:

  1. Failing to check all spark plugs and coils
  2. Replacing one coil instead of all
  3. Not performing a fuel system service
  4. Ignoring other codes that are stored along with P0303
  5. Not checking for vacuum leaks
  6. Not inspecting for damaged spark plug wires

Code P0303 FAQs

1. Is the P0303 code serious?

Yes, it is considered serious because it may cause drivability issues, and if left unaddressed, it may also cause several secondary problems in the long run. The fuel should be burnt in the cylinders, not leaked into them. 

Therefore, if the unburnt fuel flows down the exhaust, this fuel will be superheated and cause damage to other components, including the catalytic converter.

2. Can I drive having the code P0303?

No, driving with a misfiring cylinder is not recommended. If you lose power while driving, you may end up in a vehicle accident that causes you and others around you to be injured.

3. Can I prevent the P0303 code from coming back?

The best way to prevent the issue from coming back is to keep up with your vehicle's maintenance and have it regularly serviced.

4. What special tools can I use to diagnose this issue?

When diagnosing the P0303 code, you may need a few special tools. These may include a scanner tool, spark plug gap tool, a feeler gauge, and a torque wrench. You also need to ensure that you have a good set of jumper cables on hand.

5. How much is the diagnosis cost of the P0303 code?

The cost of diagnosing this issue may vary as a misfiring of a cylinder can happen for numerous reasons. However, if you need a spark plug replacement, the cost can range from $100 to $300, depending on the cost of spark plugs and labor.


The P0303 code is a serious issue that should be addressed as soon as possible as it can cause severe damage to your engine. Fortunately, there are many fixes that you can do yourself. While the P0303 code may seem daunting, you should be able to fix the issue in no time with a bit of patience and elbow grease! 

So how did it go for you? Please leave any questions or comments below! We hope this guide has helped get your P0303 code resolved!