Engine misfires are one of those car problems that can sneak up on you. One day your vehicle runs fine, and the next it’s shaking at idle, losing power, or flashing that check engine light. Ignoring it can lead to bigger and more expensive problems, but fixing it isn’t always as complicated as it seems.
This post explains everything you need to know about how to fix an engine misfire, including its causes, symptoms, tools you’ll need, step-by-step diagnosis, and how to prevent it from happening again.
What Is an Engine Misfire, Really?
An engine misfire occurs when one or more of your engine’s cylinders fail to ignite the fuel-air mixture at the right time. This means that the combustion process (air, fuel, spark) inside that cylinder either doesn’t happen at all or is incomplete.
For your engine to run smoothly, each cylinder must receive the correct amount of fuel, air and spark, and all must be perfectly synchronized. If any part of that equation is off, combustion doesn’t occur properly, resulting in a misfire.
A misfiring cylinder disrupts your engine’s timing and balance, causing your vehicle to feel jerky, rough or underpowered, especially when idling or accelerating.
Recognizing the Signs of an Engine Misfire
Before we discuss how to fix an engine misfire, it’s important to recognize the symptoms. Many people drive for weeks and don’t realize something is wrong, thinking it’s just “old car behavior.” But knowing what to look for is important.
You may notice a rough idle – your engine vibrates more than normal when the car is at rest. During acceleration, you may notice a lag or stumble, as if your car doesn’t have the same power as before and may also feel jerky during acceleration.
Sometimes, you’ll also hear a popping sound or smell unburned fuel from the exhaust. And of course, the most obvious sign is a blinking or steady check engine light. If you’ve noticed any combination of these, you’re probably dealing with a misfire.
What Causes an Engine to Misfire?
There are many possible reasons why an engine may misfire, and they generally fall into three main categories: ignition problems, fuel delivery problems and air/compression faults. Let’s take a look at each one separately.
- Ignition Problems: Ignition problems are the most common. Your engine’s spark plugs, ignition coils and associated wiring are responsible for creating the spark that ignites the fuel-air mixture. If a spark plug goes bad or an ignition coil fails, that cylinder won’t fire properly.
- Fuel System Failure: Next, fuel system problems can prevent the right amount of fuel from reaching the cylinder. This can be due to a clogged fuel injector, a failed fuel pump, a blocked fuel filter or incorrect fuel pressure. Without enough fuel, combustion is incomplete or doesn’t occur at all.
- Air and Compression Problems: Finally, problems with air intake or engine compression can also cause a misfire. For example, a vacuum leak can throw off the air-fuel ratio. Similarly, if your engine has low compression in one cylinder because of worn piston rings, valve damage or a leaking head gasket, the combustion process won’t happen efficiently — or at all.
- Sensor, Wiring and ECU-Related Failures: In some modern vehicles, a misfire can also be caused by a faulty sensor or bad ECU. If your oxygen sensor, MAF sensor, crankshaft position sensor or camshaft sensor feeds incorrect data to the engine control unit, it can cause a misfire even when the mechanical parts are working properly.
NOTE – Incorrect timing between the engine’s crankshaft and camshaft can often lead to engine misfires. When these components are not properly synchronized, combustion becomes unstable, causing rough performance and misfiring.
Tools You’ll Need to Diagnose and Fix a Misfire
Before jumping into the fix, gather a few essential tools that will make diagnosing easier and more accurate:
- OBD-II Scanner: This tool plugs into your car’s computer and retrieves error codes. Misfire-related codes start with P0300–P0308.
- Spark Plug Socket and Ratchet: Needed to remove and inspect spark plugs.
- Fuel Pressure Gauge: Helps check whether your fuel pump and regulator are working correctly.
- Compression Tester: Measures the compression in each cylinder to detect internal engine problems.
- Multimeter: Useful for testing ignition coils, sensors, and wiring.
- MAF Sensor Cleaner or Brake Cleaner: Helps clean sensors or detect vacuum leaks when sprayed around suspect areas.
With these tools, you’re ready to start troubleshooting your engine.
How to Fix an Engine Misfire? (Step-by-Step Guide)
Fixing an engine misfire doesn’t have to be overwhelming if you follow a logical, step-by-step process. Whether you’re a DIY mechanic or simply trying to understand what your technician is doing.
#Step 1: Scan for Error Codes with an OBD-II Scanner
The very first step in diagnosing a misfire is to plug in an OBD-II scanner and check for error codes. Modern vehicles are equipped with onboard diagnostics that store trouble codes when something goes wrong.
Look for codes like:
- P0300 – Random or multiple cylinder misfire
- P0301–P0308 – Specific cylinder misfires (e.g., P0302 = Cylinder 2)
Also pay attention to related codes, such as:
- Oxygen sensor faults
- Air intake or fuel trim issues
- Crankshaft/camshaft sensor errors
These codes help point you in the right direction and save time by narrowing down the problem area.
#Step 2: Inspect and Replace Spark Plugs
Spark plugs are one of the most common causes of engine misfires. Start by removing the spark plug from the misfiring cylinder.
Check for:
- Black, sooty tips – carbon fouling
- Oily residue – possible valve seal leak
- White deposits – overheating or lean fuel mixture
- Even if the plug looks okay, replace it if it’s worn or old.
- Always use spark plugs that match your vehicle’s OEM specifications for best performance.
This simple fix often solves misfire issues, especially in high-mileage or poorly maintained engines.
#Step 3: Test Ignition Coils or Wires
If the spark plugs are good, the next step is to examine the ignition system.
For coil-on-plug systems:
- Swap the coil from the misfiring cylinder to another one.
- Clear the codes and test-drive the car.
- If the misfire moves with the coil, it’s faulty and needs replacement.
For older cars with spark plug wires:
- Look for signs of damage such as cracks, brittleness, or burns.
- Faulty wires can leak voltage, leading to weak or no spark.
- Replace them if you notice any wear or damage.
A failing coil or wire won’t deliver a strong spark, which disrupts the combustion process and triggers a misfire.
#Step 4: Inspect the Fuel System
If ignition components are fine, fuel delivery could be the issue. Each cylinder depends on a consistent flow of fuel, and any disruption can lead to misfiring.
Check fuel injectors:
- Use a mechanic’s stethoscope or a screwdriver to listen for a clicking sound.
- If one injector is silent, it may be clogged or dead.
- Swap injectors between cylinders to see if the misfire follows.
- Run a fuel pressure test to confirm the fuel pump and pressure regulator are working properly.
Clogged injectors or low fuel pressure are common culprits in engine performance issues, especially in vehicles with high mileage or poor fuel quality.
#Step 5: Check for Vacuum Leaks
Vacuum leaks cause a lean air-fuel mixture, which can easily trigger a misfire. Even small leaks can create big drivability problems.
- Start the engine and carefully spray brake cleaner or carb cleaner around:
- Intake manifold gaskets
- Vacuum hoses
- Throttle body connections
- If the idle fluctuates or temporarily smooths out when you spray, you’ve likely found a vacuum leak.
Replace any cracked, brittle hoses or damaged gaskets to restore proper airflow and fuel mixture balance.
#Step 6: Clean or Replace Faulty Sensors
Modern engines rely on sensors to control timing, fuel delivery, and spark. If one of these sensors fails, it can lead to incomplete combustion and misfires.
- Clean the Mass Air Flow (MAF) sensor with MAF-specific cleaner to remove dirt buildup.
- Check the crankshaft and camshaft position sensors using a multimeter.
- If a sensor has failed, replace it. Your OBD-II scan may have already flagged it.
These components are crucial for accurate engine timing. When they go bad, the computer gets incorrect data — leading to misfires and other performance issues.
#Step 7: Perform a Compression Test (Advanced)
If none of the above steps solve the problem, you may be dealing with a mechanical issue inside the engine. A compression test will help you check the health of each cylinder.
- Remove all spark plugs.
- Disable the ignition and fuel systems.
- Screw a compression tester into one cylinder at a time and crank the engine.
- Healthy readings are usually 120–200 psi, depending on your vehicle.
If a cylinder shows significantly lower compression than the others, the problem may be:
- Burnt valves
- Blown head gasket
- Worn piston rings
Low compression means the cylinder physically can’t hold pressure — and that means no proper combustion.
Maintenance Tips to Prevent Engine Misfires
- Replace Spark Plugs on Schedule – Follow your car manufacturer’s recommended intervals — usually every 30,000 to 100,000 miles, depending on your engine and plug type. Even if they look fine, old plugs can misfire under load or high heat.
- Use Quality Fuel & Cleaners – Cheap or contaminated gas can lead to clogged injectors and poor combustion. Stick with top-tier gasoline when possible, and run a fuel system cleaner every few thousand miles to keep injectors and valves clean.
- Clean the MAF Sensor Regularly – A dirty mass airflow sensor sends incorrect data to your ECU, which can throw off your air-fuel ratio. Use MAF sensor cleaner every 10,000 to 15,000 miles to keep it accurate and responsive.
- Inspect Ignition Coils and Wires – During oil changes, check your ignition components for signs of wear — like cracks, corrosion, or brittleness. Weak coils or damaged plug wires are common causes of recurring misfires.
- Look for Vacuum Leaks – A cracked vacuum hose or loose intake connection can lead to lean air-fuel mixtures and random misfires. Check hoses periodically, especially if you notice a rough idle.
Don’t Ignore Early Warning Signs
Minor symptoms like a quick stumble during acceleration or a slightly rough idle can be early warnings of a misfire. Don’t shrug them off. The sooner you address a small issue, the less likely it is to evolve into a costly repair or a failed emissions test.
When You Should Go to a Mechanic?
Sometimes, even after you have replaced spark plugs, tested ignition coils, cleaned sensors, and swapped fuel injectors, your engine misfire still does not go away. That is when it is best to stop guessing and call in a professional.
If you have performed a compression test and found one or more cylinders with low compression, this usually indicates a more serious mechanical issue. Problems like burnt valves, worn piston rings, or a blown head gasket are internal engine failures.
These issues cannot be fixed by simply replacing spark plugs or sensors. They require specialized tools, engine disassembly, and sometimes major repairs.
Another warning sign is an electrical or computer-related problem. If your car’s ECU (Engine Control Unit) is malfunctioning, misfiring due to faulty wiring, or giving erratic sensor readings, a certified technician is your best option. These problems often need advanced diagnostic tools, such as dealer-level scan equipment, to find and fix the exact cause.
You should also consider seeing a mechanic if:
- The check engine light keeps flashing after multiple repair attempts
- The misfire comes with overheating, white smoke, or coolant loss
- You smell strong unburned fuel or hear knocking or pinging sounds
- The car goes into jerk mode or struggles to stay running
At this point, trying to fix the problem yourself may end up costing more in parts and time than a professional diagnostic would.
Even if you feel confident using tools, some causes of misfires are beyond the scope of DIY repairs. Mechanics have access to specialized equipment like compression leak-down testers, oscilloscopes, and injector flow testers that can quickly and accurately identify the issue, saving you money and hassle in the long run.
Conclusion
To fix an engine misfire, start by scanning for error codes to identify the faulty cylinder. Check and replace spark plugs, test ignition coils, and inspect the fuel injector. If the issue remains, look for vacuum leaks, clean or replace sensors, and inspect components like the EGR valve. Persistent misfires may indicate deeper engine problems such as low compression or a blown head gasket.
FAQs
Engine misfires can be confusing, especially if you’re diagnosing the issue for the first time. Here are some common questions people ask when dealing with misfire problems, along with straightforward answers to help you out:
Q. Can I drive my car with an engine misfire?
Technically, yes, but it’s not safe or recommended. Driving with a misfire can damage your catalytic converter, reduce fuel efficiency, and lead to further engine problems. If your check engine light is flashing, stop driving immediately and have the car diagnosed.
Q. What causes an engine to misfire intermittently?
Intermittent misfires are often caused by failing ignition coils, loose spark plug wires, or even sensor issues like a bad crankshaft or camshaft position sensor. They may only show symptoms when the engine is under load, during acceleration, or when the engine is hot.
Q. How much does it cost to fix an engine misfire?
- Spark plugs: $40–$100 for parts and labor
- Ignition coils: $75–$300 each
- Fuel injectors: $100–$300 per injector
- Professional diagnostics: $80–$150+
- If the issue involves low compression or internal engine damage, it could cost $1,000+.
Q. How do I know which cylinder is misfiring?
Use an OBD-II scanner to check diagnostic trouble codes. Codes like P0301 through P0308 indicate which cylinder is misfiring. For example, P0304 means Cylinder 4 is misfiring. This helps you target the issue faster.
Q. Will replacing spark plugs fix a misfire?
Often, yes. Worn or fouled spark plugs are a very common cause of misfires. Replacing them, especially if they’re overdue — is usually the first and cheapest step in fixing the problem. However, if the misfire continues after replacement, you’ll need to look deeper into coils, fuel delivery, or sensors.
Q. Can dirty fuel injectors cause a misfire?
Absolutely. A clogged or malfunctioning fuel injector can prevent enough fuel from entering the cylinder, causing a lean misfire. Using fuel injector cleaner may help in minor cases, but bad injectors may need to be cleaned professionally or replaced.
Q. What happens if I ignore a misfire for too long?
- Catalytic converter failure
- Engine overheating
- Permanent internal damage
- Failed emissions tests – It also makes your engine run less efficiently, costing more in fuel and repairs down the line.
Q. How can I tell if it’s a fuel, spark, or compression issue?
- Fuel issues: Misfire happens under load or acceleration, often with a fuel smell.
- Spark issues: Sudden stuttering or jerking, especially when cold.
- Compression issues: Misfire stays constant and may be accompanied by rough idling and low power. Use a compression tester to confirm.
Q. Can a misfire fix itself?
Rarely. While an intermittent misfire might stop temporarily, the underlying issue will remain, whether it’s a worn spark plug, weak coil, or clogged injector. It’s best to address it before it leads to more serious damage.