P0306: What Does This Trouble Code Mean And How To Fix It?

by Conner Mckay

Every car since 1996 uses a universal OBD-2 system that allows you to scan the diagnostic system. The scan will return a list of trouble codes (if there are any) that can help you to find the source of your car’s troubles. One of these codes is the P0306 trouble code, a powertrain-related code.

If you’ve come across this code recently, we’ll discuss everything you need to know about this. From what the code means, to how to fix the problem. Here’s our table of contents to help you navigate:

P0306 Trouble Code: What Does It Mean?

There are, quite literally, thousands of trouble codes that your car can register. Most are universal, meaning that the codes mean the same thing no matter what your car’s make and model is. However, some are manufacturer-specific. This means these codes are unique to a manufacturer and will mean different things depending on the car’s make and model.

As for the P0306 code, it’s a universal code and is the same no matter your car’s make and model. So, what does the P0306 code mean then? Well, the P0306 code means the car’s On-Board Diagnostics (OBD) system detects a misfire in cylinder six of the engine. Let’s break down the code to help you understand further:

The ‘P’ in the code stands for ‘Powertrain’. So, if you see this code, it means your car is experiencing a powertrain-related issue. Meanwhile, the ’03’ indicates that it’s an ignition system and engine misfire problem. The last two digits, in this case, ’06’, indicate the specific cylinder that’s experiencing this issue.

The P0306 code may or may not be accompanied by other trouble codes. If it’s accompanied by other codes such as the P0304 code, this means you have misfires in cylinder four as well. In this case, it will likely be accompanied by the P0300 which means you have multiple misfires.

If you have multiple misfires in your engine, our guide to engine misfires might be more relevant for you. This article will focus on what to do if you have a misfire in only one of your engine’s cylinders.

P0306: What Is A Cylinder Six Misfire?

To explain this, we need to quickly explain how an internal combustion engine works. In a nutshell, your car runs on a series of controlled combustion inside the cylinders. Modern cars have a four-stroke engine, and it has four stages in the cycle:

  1. Intake stroke, where the engine puts air and fuel into the cylinder.
  2. The compression stroke is when the piston inside the cylinder compresses the fuel and air mixture.
  3. The power stroke is when the spark plugs “fire” or create a spark to combust the mixture. This combustion powers the piston and through a series of shafts and gears, it ultimately powers your car.
  4. The exhaust stroke is when the engine expels the exhaust gases via the exhaust valves.

The engine does this process thousands of times per minute when you drive the car. And each cylinder has to be doing the right stroke at the right time for it to run smoothly.


A cylinder six misfire means that cylinder number six in your engine isn’t firing correctly. This will often feel like the engine is missing a beat and hesitating. If you’re accelerating, you’ll notice that the engine’s RPM isn’t climbing quite as quickly as it should.

This is not to be confused with an engine backfire, although a backfire can also trigger the P03xx trouble codes. An engine backfire is when the fuel combusts at the wrong time, often due to faulty ignition timing. Much like an engine misfire, it will also make your car feel like it’s hesitating, but it will often be accompanied by a loud popping sound.

P0306: What’s Causing This Problem?

Unfortunately, the P0306 code won’t tell you the exact cause of the problem. Trouble codes will only tell you the issue your car is experiencing, but not the source of the problem. If you see the P0306 code, here are the likely possible causes:

  1. Ignition system issues.
  2. Vacuum or fuel system leak.
  3. Cylinder compression loss.

Let’s delve deeper into each of them along with how to diagnose them and their repair costs:

P0306 Cause: Ignition System Issues

The ignition system comprises several components, mainly the spark plugs and the ignition coil. This system is responsible for creating the spark inside your engine’s cylinder which combusts the fuel and air mixture. If one of the components isn’t working, it will result in a misfire.

As mentioned, the spark plug is the device that creates the spark inside the engine’s cylinder to combust the fuel and air mixture. Meanwhile, the ignition coil is the device that delivers power to the spark plugs so that it can create a spark.

Most cars today use a “pencil” coil or coil-on-plug system, where the coil directly connects to the spark plug. However, older cars often use a distributor-type system. In this system, the coil sits remotely from the spark plugs and connects to them via an ignition cable.


Keep in mind that diesel engines don’t have an ignition system. Instead, they rely on the engine’s high compression ratio to combust the fuel and air mixture. So, if you have a diesel engine, you can ignore this section and move on to the next potential cause.

Additionally, we recommend checking your service history and finding out when the spark plugs and ignition coils were last changed. These two components will wear out over time and you need to change them periodically.

Spark plugs typically last between 20,000 to 40,000 miles, although some may last longer. Meanwhile, an ignition coil lasts for around 100,000 miles. Check your service history, and if you’re overdue for a change, then changing them should fix the problem. If you’re not overdue for a change, then here’s how to diagnose:

Diagnosing The Ignition System (Coil-On-Plug System)

This diagnosing method will help you find out which component of the ignition system (if any) is faulty. The exact steps will vary depending on your car’s make and model, so this is going to be more of a general guide. Here’s what you need to do:

  1. Check your owner’s manual to locate cylinder six of the engine.
  2. Remove the spark plug and ignition coil from the engine. You will need to remove the ignition coil’s connectors in the process.
  3. The next step is to swap the spark plug and ignition coil from cylinder six with ones from other cylinders.
  4. As an example, swap the spark plug from cylinder six with the spark plug in cylinder four. Meanwhile, swap the ignition coil from cylinder six with the coil from cylinder two. Afterward, reconnect the coil connectors.
  5. Next, clear the trouble code from the OBD scanner. Turn on the engine, and let it run for a few minutes, or drive it until the check engine light comes back on.
  6. Once the check engine light appears again, scan the OBD system with the OBD scanner. If you still have the P0306 code, then something else is causing cylinder six to misfire.
  7. If you have a different code, then a different cylinder is now misfiring. In our example, if cylinder four is misfiring (P0304) then the spark plug is faulty since we put the spark plug from cylinder six into cylinder four. If cylinder two is misfiring (P0302), then the ignition coil is faulty.

We recommend watching the video above, it will give you a better idea of how to do the process. The engine in the video above is a four-cylinder engine, but the diagnosing process is largely the same.

Diagnosing The Ignition System (Distributor-Type Coil)

There are two types of distributor-type coil systems, but regardless of which one your car has, the process is largely the same. Instead of swapping the ignition coils, you’ll be swapping the ignition cables. Here’s how to do it:

  1. Disconnect the ignition cables from the engine and the distributor. Note that you will need to memorize or mark the connection on the distributor concerning the cylinders. Each cylinder has to be connected to the right connection on the distributor.
  2. Afterward, swap the spark plug from cylinder six with cylinder four. And swap the ignition cable on cylinder six with the ignition cable from cylinder two. You can swap it with any other cylinders you like, this is just an example.
  3. Clear the OBD codes from the car, then turn the engine on to trigger the check engine light again.
  4. Turn off the car, and scan for trouble codes. Much like the coil-on-plug system, follow where the misfire happens.
  5. If it still happens at cylinder six (P0306) then you have a different problem.
  6. If the misfire now happens at cylinder four (P0304) then you have a faulty spark plug. Meanwhile, misfire in cylinder 2 (P0302) then you have a faulty ignition cable. Multiple misfires mean that both components are faulty.

Ignition System Replacement Costs

The replacement cost will vary depending on which component you need to replace. Spark plugs are the cheapest and easiest to replace. A set of quality spark plugs is usually between $60 – $100. You can buy them individually, but we recommend changing all of your spark plugs at the same time.

Labor for a spark plug change will cost around another $80. But since you’re doing the diagnostics yourself, we’re assuming you’re not going to worry about labor costs. Meanwhile, ignition coils are more expensive, but not too difficult to replace, especially for coil on plug systems.

A single ignition coil for a coil-on-plug system will cost around $60 – $80. However, this can vary greatly depending on your car’s make and model. And much like spark plugs, we recommend replacing them all at the same time if the cost isn’t out of your budget.

Meanwhile, the distributor-type coil replacement cost varies depending on your car’s make and model. But it’s usually around $300 – $400 for the whole distributor unit, while the labor is around another $90. If you have a bad ignition cable, then it’s usually around $30 to replace each of them. Read our guide to ignition coil replacement cost to learn more.

P0306 Cause: Vacuum Leak Or Fuel Issue

If you don’t have any issues with your ignition system, the next possible cause for your cylinder misfire is a vacuum leak and/or a fuel system issue. As mentioned, your engine requires a mixture of fuel and air to combust and run the engine.

This mixture has to have the right air-to-fuel ratio, otherwise known as the stoichiometric ratio. If this mixture is too rich (too much fuel) or too lean (too much air), the engine can misfire. Additionally, it can lead to a myriad of other problems, this is why you don’t want to ignore a misfiring engine. Here’s how to diagnose them:

Diagnosing Vacuum Leak

A vacuum leak is when one of the vacuum hoses is leaking. This can either reduce or increase the amount of air your engine takes in. As a result, the engine’s sensors will get wrong readings and messes up the air-to-fuel ratio.

If you have a vacuum leak, there’s a possibility your engine will misfire. Another symptom you might notice is a high-pitched hissing noise coming from the engine when your rev the engine. Anyway, here’s how to diagnose it:

  1. Visually inspect the car’s vacuum hoses. If any of the hoses seem broken or have torn apart, then that’s your problem and you will need to replace them.
  2. If you can’t find any physical damage, a popular method is to spray carburetor cleaner onto the vacuum hoses. However, keep in mind that this is a fire hazard, so be very cautious, do it while the engine is cold, and prepare a fire extinguisher.
  3. Turn on the engine.
  4. Spray the carburetor cleaner to areas where the vacuum hoses connect to.
  5. If the engine’s idle fluctuates or it shuts off completely, then that’s where your vacuum leak is coming from.

The video above from Eric The Car Guy should give you a better idea of how to perform this vacuum leak test. Finding a vacuum leak may be difficult, so we recommend leaving this to a professional if you don’t feel up to the task.

Note that other issues with the air intake system may cause the engine to misfire, such as a faulty Mass Air Flow (MAF) sensor or a leak in the intake pipes. However, these issues will usually cause multiple misfires, not just on a single cylinder.

Diagnosing The Fuel System

If your air intake system is fine, then your engine may be experiencing issues with the fuel system. The fuel system consists of many components, but in this case, the likely culprits are either the fuel filter or the injectors.

If the fuel filter is clogged, then your engine won’t get enough fuel. As a result, this messes with the fuel-to-air ratio and will lead to a misfire. However, a fuel filter problem is more likely to cause misfires in multiple cylinders, rather than just one cylinder.

Instead, the more likely culprit is the fuel injector. In most cases, each cylinder of the engine has its own fuel injectors. If the injector for that cylinder is faulty, it will cause misfires in that cylinder, but not in other cylinders. Checking them is relatively easy, and all you need is a long screwdriver:

  1. Open your engine hood and locate the fuel injector for cylinder six.
  2. Start your engine.
  3. Put the screwdriver against the back of the fuel injector.
  4. Put your ear against the handle of the screwdriver and listen for a clicking noise.
  5. A good fuel injector will make a constant and consistent clicking noise. If there’s no clicking noise or it’s inconsistent, then this is a sign that you have a faulty fuel injector.

If the fuel injector isn’t working, then you’re essentially starving your engine of the fuel that it needs. As mentioned, this can mess up the air-to-fuel ratio which results in engine misfires. Keep in mind that the injector may not be working because of a faulty electrical connection. We recommend watching the video above from ChrisFix to learn more.

Replacement Costs

So, how about the repair costs? If you’re replacing the vacuum hose, each of them will cost around $140 – $190 to replace including labor. As for the fuel filter, it will cost between $100 to $300 to replace including labor.

Note that a fuel filter should be changed after about 25,000 miles, check with your owner’s manual to find the exact interval. You can learn more about this in our guide for fuel filters.

The fuel injectors are going to be the most expensive ones to replace. Depending on your car’s make and model, a set of fuel injectors cost between $600 – $1,200, with around another $200 for the labor cost.

The good news is that you may be able to clean the injectors to solve the problem. This will cost between $50 – $100 to do professionally, or you can also use a DIY fuel injector cleaning kit. We have a great guide on fuel injector repairs and we’re sure you’ll find it helpful.

P0306 Cause: Cylinder Compression Loss

The last – and most expensive – cause for a single-cylinder misfire that can cause a P0306 trouble code is compression loss inside said cylinder. The engine’s cylinders are sealed and require a certain amount of compression to work properly.

If the cylinder leaks, then it won’t be able to achieve the necessary compression rate. There are several possible causes for a compression leak, and we’ll get into that later. However, they are all equally expensive and you should prepare yourself for a hefty repair bill.

To find out if you have a compression loss, you will need to do a compression test:

Diagnosing Cylinder Compression Loss

To do this test, you will need an engine compression tester. They usually cost around $15, and while it’s mostly universal, it’s a good idea to make sure that they will work for your car’s make and model.

Once you get hold of the tester, here’s what you need to do to perform the test:

  1. Cut off the fuel supply and ignition system, you don’t want the engine to run. The easiest way is to locate the fuse for your car’s fuel pump and ignition system and disconnect the fuses.
  2. Pull out the ignition coil and spark plug from cylinder six, since this is the cylinder that’s misfiring. You can do this to other cylinders as well if you have misfires in multiple cylinders.
  3. Insert the compression tester into the spark plug’s port. Make sure to tighten the compression tester into the port.
  4. Next, you’ll need a friend to crank the car. Have them crank the car, while you pay attention to the compression gauge. Keep cranking the car until the gauge’s needle no longer climbs, and take note of the peak compression.
  5. A cylinder’s compression should be somewhere between 100 to 150 psi. If it’s below that, then there’s a leak inside the cylinder. You can also do the other cylinders if you’d like, and a healthy engine should have no more than 10% variation between the highest and lowest readings.

As the video above mentions, there are several possible reasons for cylinder compression loss. We’ll explain more about them as well as the potential repair costs:

Cylinder Compression Loss Repair Costs

Cylinder compression loss can happen because of these things:

Regardless of which one is causing your engine’s cylinder to lose compression, they are all equally expensive to repair. It’s because they are internal components, and to replace them, you essentially need to dismantle and then rebuild at least half of the engine.

Damaged valves are usually the cheapest ones to repair, costing between $900 – $1,800. However, the other causes will require you to either rebuild or replace the engine in your car entirely. You’ll need to prepare at least $2,500 for this, and don’t be surprised when your mechanic quotes somewhere between $4,000 – $5,000 for the repairs.

Facts about Diagnosing Code P0306 for Misfires in Cars

  1. Code P0306 indicates that cylinder 6 is misfiring, and the misfire usually occurs when the ignition timing is wrong.
  2. The most common reason for a misfire is a malfunctioning ignition system, fuel system, or internal engine failure, such as faulty or worn-out spark plug coil packs.
  3. There are various reasons for a misfire, including cracks in insulation, an open circuit in the ignition coil, no ignition spark, leaky valves, low compression, clogged fuel injectors, and more.
  4. Other potential reasons for error P0306 include compression difficulties caused by defective rings and valves or oil leaking from the valve cover into the spark plug openings.
  5. Symptoms of a misfire include a bright or blinking check engine light, rough engine acceleration, poor fuel economy, and gasoline smells.
  6. To diagnose a code P0306, drivers can use a reliable OBD2 scanner to record error codes and freeze frame data, visually inspect connectors and wiring, and check the fuel injector, ignition coils, and related wiring.
  7. Common mistakes in diagnosing code P0306 include failing to check for loose electrical connectors, not isolating and identifying failed components, and not checking for a connection between the coil and the cylinder.
  8. It is important to fix misfires immediately to prevent more serious consequences.
  9. More cylinders in an engine generally imply more power, with most engines having four, six, or eight cylinders.
  10. Drivers may not notice any problems while driving during code P0306, but the engine may have poor fuel economy or rough idling, and a lack of power.

P0306 Trouble Code: Wrap Up

So, to summarize, the P0306 trouble code means that there is a misfire in cylinder six of your engine. A misfire happens when one or more cylinders aren’t “firing” or igniting the air and fuel mixture inside it. An engine misfire often feels like the engine is hesitating and feels rougher than usual.

In most cases, a P0306 code or other engine-misfire-related code is caused by a faulty spark plug or ignition coil. Both of these components are quite easy to replace and relatively affordable.

However, it may also be caused by a vacuum leak or an issue with the fuel system. This will prevent your engine from getting the correct air-to-fuel ratio and cause misfires.

In severe cases, the misfire may be caused by damage to the engine block or internal components. This will be very expensive to repair as you will either need to rebuild or replace the engine. Hopefully, this has been a helpful guide for all your questions about the P0306 trouble code.

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