How to fix an engine misfire

In Troubleshooting by Josh S2 Comments

Is your car suffering from the trouble codes P0300, P0301, P0302, P0303 or P0304? This DTC trouble code is coming from a misfire on one or several cylinders. Why should you pay a mechanic to fix this simple DTC code when you can fix them yourself at home after you have checked out this article.



I’m working as a car technician. I’m spending my days to find and fix problems like this and I will help you out to get rid of your misfires in this article. The normal response I get from the customers is that they have no idea at all what the mechanic is doing to their car. The mechanics could tell the customers whatever they want and the customer will still pay for it. Of course, it is a good thing that you give trust to your mechanic, but some of the mechanics are abusing the situation. That’s why I’m writing this articles to give you more knowledge about how to save some money and fix your car at home.

Many faults are easy to diagnose yourself at home if you have the interest, and a misfire is one of those problems. If you learning more about your car and how it’s working, you can have a bit more knowledge about what the mechanic is actually doing on your car and you could save a lot of money. Now let’s start this article about how to fix misfires at home.


What is a misfire?


This is a good picture of how your pistons and crankshaft are moving inside the cylinder when your engine is running. The pistons are pushed down by an explosion inside the cylinder. When the piston is pushed down, the crankshaft is spinning. The engine is working in four steps, that’s why this engine type is called a four-stroke engine.

  1.  Piston goes down, filling the cylinder with an air-fuel mixture from the intake
  2.  Piston goes up, compressing the air-fuel mixture to a high pressure
  3.  The ignition from the spark plug is igniting the air-fuel mixture and the explosion is pushing the piston down and rotating the crankshaft
  4.  The piston goes up, emptying the burned air-fuel mixture through the exhaust pipe.
  5.  Repeating the process from step 1

That’s the function of a four-stroke engine that is fitted in the most cars today. There are some older cars that are using a 2-stroke engine but I will not go through that procedure in this article. If you are interested in how a two-stroke engine works, you can check out this article:  Two-stroke engine

A misfire occurs when ONE or more of this stages is wrong or missing

  • Too lean or too rich air-fuel mixture
  • Bad ignition spark / wrong timing the ignition spark
  • Low compression / air-fuel mixture is leaking out
  • The timing of the inlet/outlet of the air-fuel mixture is wrong

Now you know the basics of how a cylinder in a car engine is working and in what steps the misfire would possibly occur. With that knowledge, it’s alot easier to find the fault that is making your misfires. As you see, in the theory there is not a lot of things that could cause a misfire. But when you are starting to diagnose your car, you will realise that it is not always that easy to find the problem as it looks. Continue your reading to learn how to diagnose and how to fix your misfires at home


How does the engine control module know when a misfire occurs?

The engine control unit can detect a misfire in some different ways, depending on what car model and engine you have. The engine control module is using a lot of sensors to know when to ignite the spark plug when to inject the fuel into the cylinder and to optimize the air-fuel mixture. To detect a misfire, the engine control module is often using the crankshaft sensor to detect a misfire.

The crankshaft sensor is measuring the position of the crankshaft and calculates the rpm ( Revolutions per minute ). The crankshaft sensor uses the camshaft sensor to detect which of the cylinders that are up in the top and are ready to be ignited. When the pistons are pushed down, there will be a little speed increase of the crankshaft. If the crankshaft sensor is not recognizing that increase of speed, the engine control unit will save a trouble code on that cylinder which the misfire occurred on.

Sometimes the engine control unit cannot detect which cylinder the misfire occurs on and it will put the DTC code P0300 (random misfire). Some engine control modules are using the resistance of the ignition coil and when an ignition does not occur, the engine control unit measure it through the wirings and it would result in a DTC code. This function is not as normal as the crankshaft sensor detecting thought.


How to diagnose and fix a misfire

If you have read the article from the beginning, you now know what could cause a misfire. If you jumped directly down here I will write the possible causes fast. A misfire occurs if one of this are missing/wrong timing

  • Ignition
  • Air-fuel mixture
  • Compression

To diagnose a misfire or several misfires, we have to go through all steps to check that everything is ok. But some faults are more common than others and it could save you a lot of time to start from the right direction. As I told you before I’m working as a car technician, diagnostic technician. I will write in some steps of how I do to detect a misfire as fast as possible when I’m solving this issues. First I will list the most common problems when a misfire occurs.

Most common causes of a misfire

  1.  Bad ignition coil/distributor if you have an old car
  2.  Intake leaks
  3.  Bad Spark plug
  4.  Low fuel pressure
  5.  Injector problem
  6. Low compression/damage inside the engine


1. Read the DTC code memory

When you think that your engine is suffering from misfires, the first thing you always should do is to read the DTC trouble code memory. If you want to do it yourself you can check out this tool. It’s not too expensive and works with the most car brands, always make sure that it’s working with your car before you buy it! Check it out on amazon

If you find any DTC codes in the DTC code memory. Write them down and clear the dtc memory, you are going to need them in the next step.

2. Use the information you got

Now when you know what DTC codes you have. Search for some information of what the dtc code is telling to you. You should start in different ways in your diagnose, depending on what trouble codes you got.

Case 1: You get a lot of DTC codes of misfires on different cylinders together with an air/fuel mixture related trouble code.

For an example. If you have two or more DTC codes that could seem to be related to each other

  • P0171 – System Too Lean ( Bank 1 )
  • P0300 – Random misfire
  • P0301 & P0302 & P0303 & P0304 etc. Several misfire codes

In this case, if the engine is running roughly or bad on specific RPM, you should start to fix the air/fuel mixture related trouble code and that will probably cause the misfires too. If you have an air/fuel mixture related code together with the misfire code, always search for information about the code related to air/fuel mixture trouble code first. There are cases where you get air/fuel mixture related trouble codes due to misfires also, but it’s not as common.

There are some general common causes that you could check for in this case. Start by checking for air leaks around the intake, check for cracked hoses. You can use brake cleaner or start gas to spray carefully around the intake when the engine is at idle. If the RPM is raising, you have an intake leak somewhere there. Remeber that those sprays are burning very easily so be prepared with a fire extinguisher in case it ignites.

If you can’t find any intake leaks, the next step is to start researching about the air-fuel mixture related trouble code. You could use Google to find the possible causes and solutions to this trouble codes or you can ask us at our homepage

Case 2: You get a trouble code on a specific cylinder over and over after you clear the memory

If you are getting a trouble code on a specific cylinder all the time or just trouble codes without another related trouble code, you could continue this guide. This problem is much easier to fix and I will talk more about it in the next step.


3. Check the ignition

Now it’s time to start to check the ignition. Because in the most cases the problem is caused by a failing ignition, that’s the step we are going to start with. If you have an older car with distributor ignition. Always start by checking inside the distributor, it’s very common that the distributor is worn out.

If you are feeling that the engine is running rough at idle and if you have a newer engine with separate ignition coils, you could let the engine idle and then unplug one coil at a time. Remember that these ignition coils have very high voltage and you should be really careful.
When you unplug one ignition coil and you can’t feel any difference in the way the engine is running, you have found which cylinder that is misfiring.

You can use the same procedure with older cars that are using a distributor ignition by unplugging the ignition cables until you find out what cylinder the car is not running on. When you have ignition cables, don’t use this method while the car is running if you are not very careful and uses the right tools. If you are unplugging the cables with your hand without any safety you will get a lot of voltage through your body. Always use either the right tools or unplug them with the engine off.


Now when you know for sure which cylinder the car is not running on. You could try to move the ignition coil/ignition cables between two cylinders. If the problem is moving to the other cylinder by doing the same procedure as above when you are unplugging the coils, then you have found the problem. Replace the ignition coil or ignition cable

If the problem is still on the same cylinder, you could do the same with the spark plugs. Move the spark plugs between two cylinders and see if the problem is moving. If the problem is moving, Replace the spark plugs. Now if you have tried to move the spark plugs and coils between the cylinders but the problem are still there in the same cylinder, you have to check if you have spark, I would recommend this video for this:


If you don’t have any spark on that even if you are moving coils and spark plugs between the cylinders, you have to get a wiring diagram for your engine to start to check if you have power and ground etc to your coil. This step is more complicated and I will not write about it here. If you have any questions ask us

If your spark is OK, but the engine is not running on that cylinder, go to the next step.


4. Check for intake gasket leaks or other intake leaks

To do this, you can let your engine idle and listen around the intake gasket if you can hear some strange noises from leaks. You can use some starter spray or brake cleaner and spray around the intake gasket if the engine revs up while you are doing that. You got an intake leak there and you have to fix that.
Always bring a fire extinguishers because if the spray or brake cleaner is igniting, it will burn really well. Trust me, I’ve tried that too 😉

Intake leaks do not often make the engine to not run at all on that cylinder, it would have run roughly on that cylinder or other cylinders instead. But it’s easy to check and it could be a possible cause. If you didn’t found any intake leaks, go to the next step.

5. Check the compression

Now when you know that the spark is okay but it’s not running on that cylinder, it’s time to check the compression on all the cylinders. To check the compression you can use a tool like this: Check it out on Amazon. I will not go into details here of how to make a compression test. Watch this video instead, he will explain in details of how to make a compression test.



If you see that you have very low compression on the cylinder that the car is misfiring on, put some oil in the cylinder with low pressure and test again. If you still have low compression, there is not much more to do, you have to go deeper into your engine. You have to check the valves, valve shims and the piston rings.  You can also check the camshaft timing, but in that case, you often got low compression on all cylinders. If your compression is OK it’s time to go to the next step.

6. Check the fuel

Now there is only one possible cause left, your engine is not injecting any fuel on that cylinder.
In this case, where you are not getting any fuel to only one cylinder, you have a new car with injectors. Because if you had an old engine with a carburettor it would not cause the engine to not run on just one cylinder.

Start by unplugging one connector to the injector on that cylinder that is not running. Check if you got 12-volt power on one of the wires with the ignition on. Check with a multimeter. The other wire is grounded by the engine control unit, and not that easy to check if it’s working properly. You could use a diode light to see if it’s coming any ground on that wire when the engine is running, but I recommend that you use an oscilloscope and you will probably not have one at home. You can also check the fuel pressure, but in that case, it would probably run badly on several cylinders.

If you got 12volt to the injector and if you are reaching the injectors easily, you can use the same procedure here as with the ignition coils. Switch between another cylinder to see if the problem is moving. If your problem is still there you have to get access to an oscilloscope and check the ground signal from the engine control unit.

Now, I hope that you have found your problem. If not then you have to go through this guide step by step one more time and if you really can’t find the problem. Contact us and we will try to solve your problem and see if there is some step that you missed.



  • Ignition coils are the most common problem if you have a misfire
  • Intake leaks is a common issue if you have an air mixture related DTC code also
  • If you have an air mixture trouble code, always start by checking that code first

I hope that you have learned something and that you solved your questions about misfires. If you have more questions or want us to add or edit anything in this article, you can either comment below down here or you can send us a question at our homepage

See you in the next article!

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  1. My 1997 Sebring run good , as long as I stay under 65 . If I get on the highway at 70 , It does not take long to get the check engine light. Code says multiple misfire’s . but I don’t feel or hear any misses . Was told it might be the fuel sending unit . Inject not getting fuel , so cylinder does not fire , so get misfire code. Is this possible ?

    1. Author

      Yes it sounds like lean mixture caused by either low fuel pressure or vacuum/boost leak. I would start with checking for any leaks around the intake and connect a fuel pressure gauge if possible!

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