Informational Guide

What Is An Engine Misfire?

What is a misfire? We will look at what causes this engine performance problem and what you can do to fix it.

You are driving down the road, and you feel something off with the performance of your vehicle. It might be a hesitation or sputtering, but it is commonly known as a misfire. What is a misfire? We will look at what causes this engine performance problem and what you can do to fix it.

Worn-out or faulty spark plugs

One of the most common reasons for a misfire is faulty spark plugs. However, this is just one part of the ignition system that can go bad. With a typical ignition system, you might also face a faulty crankshaft position sensor, control module, spark plug wires, or coil packs that have failed, resulting in a misfire.

Air to Fuel Ratio Imbalance

In the engine, air and fuel are mixed together. This mixture is ignited by the spark plugs to create an explosion that keeps the engine running. If there is an issue with this air/fuel ratio, you might experience a misfire. The most common causes for imbalance include a vacuum leak or defective fuel injector.

Lack Of Compression

Any engine mechanical issue can lead to a misfire, especially when it involves the cylinder. It’s vital for the cylinder to remain sealed for proper compression. So, if your vehicle is suffering with a head gasket problem or leaking valves, the compression will be off, leading to a misfire.

Emissions equipment issues

Emissions equipment further works with the air/fuel mixture, with a fault leading to a misfire. The trouble could stem from issues with the positive crankcase ventilation (PCV) system or the exhaust gas recirculation (EGR) system.

Sensor and module problems

Modern vehicles are filled with sensors for every system. However, there are some working directly with the PCM, determining some of the most critical functions, including spark timing and fuel delivery. If one sensor malfunctions, an engine misfire can follow. Sometimes, even a faulty PCM can cause your problems.

Damaged control circuits

The vital components of your vehicle, such as the ignition coil packs and sensors, all connect through electrical circuits. If there is a fault in one of the circuits, you might notice an engine misfire. It could be something as simple as a loose connection or damaged wiring.


Signs of Engine Misfire

Rough Idle

Many people start to wonder - what is a misfire in a car – when it begins to idle roughly. This is a surefire sign that something is wrong with the fuel or ignition system. If you are sitting at the stoplight and notice the engine running poorly, you likely are dealing with a misfire.

Rough/Poor acceleration

If the car jerks when you accelerate or seems to lack power, this is another sign. When the air/fuel mixture isn’t right, the acceleration suffers. It can also lead to severe performance issues, leaving you wondering if you will even make it home.

Vibrations & Engine Noises

Your ride should be enjoyable, not filled with violent vibrations or loud engine sounds. Yet, this is exactly what happens when dealing with an engine misfire. Sometimes, the backfire sound occurs because of the unburnt fuel being pushed out through the exhaust system.

Check engine light

If your Check Engine Light comes on, you know that something isn’t right. While this light doesn’t tell you what the problem is, it does alert you that something is wrong. What is a misfire code? It is a trouble code that can be read with a scan tool, giving you a better idea of what’s wrong.

Engine Stalling

We already discussed how the misfire could create performance issues, but in extreme cases, it can also lead the engine to stall. There’s nothing enjoyable about getting stranded in the middle of the road with a vehicle that stops running.

Gasoline Smell

When the engine misfires, it can lead to a gasoline odor in your car. Whether you are smelling gasoline or a sweet fragrance reminiscent of antifreeze, it’s important that you have your vehicle looked at right away.

Unusual Exhaust

What is a cylinder misfire and how does it relate to the exhaust system? Anytime that the engine is misfiring, it can cause the fuel to burn incorrectly, which results in more exhaust fumes. You might notice a dark black exhaust cloud trailing behind the vehicle.

How To Diagnose A Misfire

Think about the scenarios in which the engine misfire occurs

What is a misfiring engine and what is causing it? That is what you are trying to figure out. Start by making notes about what’s occurring. Does the misfire only happen when the engine is cold or also when it is warmed up? Does this trouble occur during acceleration only or whenever you are driving? These are important factors to know as you continue diagnosing the misfire.

Use a scan tool

You can use an engine code scanner to hunt down the issue. Ideally, you want one that graphs data, so you can tell what cylinder is misfiring. If you don’t have a scan tool, you can visit many auto parts stores for a free scan.

Check spark plug and ignition coil

With a graphing scan tool, you will pinpoint which cylinder is giving you trouble. This is the cylinder you want to check the spark plug or ignition coil for trouble. If you don’t have a graphing scan tool, you will need to test each cylinder by swapping out these parts individually. There isn’t much difference between learning what is a cylinder 3 misfire versus what is a cylinder 1 misfire, other than where it is occurring.

Check the Fuel System

You will have to do a full check of the fuel system and injectors if nothing else seems to cause the problems. However, some engine scan tools will also pinpoint these problems, such as a faulty O2 sensor.

Check the Vacuum Systems

If everything else has been checked, you want to look at the vacuum systems. When you perform cylinder leak and compression tests, you need specialized tools. In these cases, you might prefer to visit a qualified mechanic.

car engine

How To Fix A Car Misfire: Expert Guide

If you have an engine misfire or engine codes P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, or P0308, you will want to follow these steps for repair.

  1. 1
    Look for Check Engine Light. Use your OBDII scanner to read the codes and repair the issues.
  2. 2
    Check cylinder temperatures. If you have an infrared temperature gun, you can check the cylinder temperatures. Scan the port that comes out of the exhaust manifold from each cylinder. If one is significantly cooler than another, it might not be firing. Of course, this only works if the engine is currently misfiring, not if it is sporadic. 
  3. 3
    Repair any vacuum leaks. If there is a broken vacuum line, you want to replace it immediately.
  4. 4
    Replace a faulty fuel injector. With a test light, you can check each fuel injector. If the light comes on, there is power going to the injector. However, you will want to replace any that are lacking power or look for an underlying reason why it’s not getting power. You may also be able to clean the system with some fuel injector cleaner added to your next tank of gas.
  5. 5
    If errors indicate a faulty O2 or mass airflow sensor, replace it.
  6. 6
    Inspect the spark plugs and replace them if needed. This is a regular maintenance task that should be performed during your scheduled tune-up
  7. 7
    Perform a compression test and repair any issues you find, such as replacing a head gasket.

If all else fails, it might be time to visit your local mechanic. 

People Also Ask (FAQs)

How can engine misfires be avoided?

To avoid all engine performance issues, you want to perform regular tune-ups. Follow the recommended maintenance schedule outlined in your owner’s manual.

Can I drive with an engine misfire?

It’s never wise to drive with a car problem, especially one that causes your car to stall or sputter, leaving you vulnerable for an accident. Plus, prolonging necessary repairs only leads to more damage and costly repairs down the road.

Will the injector cleaner fix a misfire?

If you have a clogged fuel injector causing problems, a fuel injector cleaner might solve the problem. Otherwise, it will do nothing to repair other causes such as compression concerns or a faulty spark plug.

Will a tune-up fix a misfire?

It can repair a misfire if the trouble is caused by a faulty spark plug or other component swapped during the tune-up.

How much is it to fix a misfire at a mechanic shop?

It depends on what is causing the problem. Swapping out a spark plug will cost far less than getting a head gasket replacement.


What is a misfire? It's the lag, or lack of acceleration felt when something is malfunctioning in the vehicle engine. All trouble can be serious and should be looked at right away. Follow our steps to gain a smooth ride once again.