How to Diagnose and Fix an Engine Misfire

An engine misfire occurs when one or more cylinders fail to fire correctly due to spark, fuel, or air issues. Common causes include worn spark plugs, faulty ignition coils, clogged fuel injectors, or airflow restrictions. Diagnosing the misfire promptly prevents engine damage and restores smooth performance. O’Reilly Auto Parts provides the tools and parts to help you complete these checks with confidence.

When your engine is running properly, all cylinders fire evenly to produce smooth power. A misfire disrupts this balance, leading to rough idling, hesitation, or reduced performance. This guide explains how to identify symptoms, pinpoint the cause, and fix an engine misfire safely using the right diagnostic tools and replacement parts.

Ignoring an engine misfire can cause serious damage over time. Unburned fuel entering the exhaust can harm the internal elements of the catalytic converter, and uneven engine loads strain internal components, reduce fuel efficiency, and increase wear. Promptly diagnosing and repairing misfires protects your vehicle, limits the risk of more costly repairs, and keeps your engine running safely and efficiently.

Misfires often cause engine performance issues that can be felt while driving. While some minor or intermittent misfires can go undetected, the most common signs of an engine misfire include:

• Rough idling or shaking at a stop
• Loss of power or hesitation during acceleration
• Unusual exhaust noises or sputtering
• Check Engine light illumination with a stored diagnostic trouble code (DTC)

These symptoms indicate incomplete combustion and should be addressed immediately to prevent further engine damage. If your Check Engine light is on, O’Reilly Auto Parts can provide a Free Check Engine light test with our O’Reilly VeriScan program, available at any store.

Diagnosing a misfire involves inspecting and/or testing spark, fuel delivery, and airflow components to identify the affected cylinder and its underlying problem.

Step 1: Check for Spark

Reason: Spark is required to ignite the air-fuel mixture.
Use a spark tester on each cylinder to ensure power is present from each coil. If no power is detected, inspect the spark plug, ignition coil, or plug wire, if applicable. Replacing faulty spark plugs or coils often resolves the issue.

Step 2: Inspect Ignition Components

Reason: Consistent spark requires all ignition components to function together.
Check for worn or fouled spark plugs, damaged or worn wires, or failing coils. Replace any components showing wear or failure. You can learn more about how to test ignition coils in our How-To Hub.

Step 3: Check Air Intake Components

Reason: Proper airflow ensures correct fuel mixtures, and low or miscalculated air flow may lead to lean- or rich-running conditions.
Inspect the engine air filter for blockages and replace it if needed, clean the throttle body, and check the mass air flow sensor, cleaning it as necessary. Restricted airflow can lead to misfires, and inaccurate readings from the MAF sensor may cause poor fuel mixtures which could lead to spark plug fouling or rough running conditions, including misfires.

Step 4: Test the Fuel System

Reason: Correct fuel pressure and delivery are essential for combustion.
Check fuel pressure, inspect the fuel filter, and test fuel injectors. You should also check for leaks near the fuel injectors that may be limiting the amount of fuel reaching the cylinder. Clogged injectors or low fuel pressure may prevent proper cylinder firing. Learn more about how to use a fuel pressure tester with our How-To resources.

Step 5: Test Compression

Reason: Poor compression can prevent the fuel mixture from reaching appropriate pressures for combustion.
Use a compression tester to check the compression levels in each cylinder. The compression readings should be close together across the engine, and if one is especially low, this could point to damaged piston rings, leaking valves, or other issues. Learn more about how to use a compression testerin our How-To Hub.

Spark plugs and ignition coils work together to ignite the air-fuel mixture in your engine, but they serve different roles in the ignition process. Understanding how each component functions can help you diagnose misfires more accurately.

Understanding these differences helps prevent unnecessary part replacements and speeds up diagnosis.

Seek professional assistance if:

• The misfire persists after basic diagnostics
• Specialized tools are required
• Internal engine damage, compression loss, or electronic faults are suspected

Modern vehicles often require advanced diagnostic equipment for accurate repair, especially if the issue is related to inaccurate signals being passed to or from the engine control module.

Can I drive with an engine misfire?

Driving with a misfire is not recommended, as it can damage the catalytic converter, cause internal engine damage, and reduce engine performance and efficiency.

Will a misfire trigger a Check Engine light?

Yes, most engine misfires will trigger a Check Engine light and store a diagnostic trouble code that can help identify the issue. You can learn more about common misfire codes in our How-To Hub.

Can bad fuel cause an engine misfire?

Yes, contaminated or low-quality fuel can disrupt combustion and lead to engine misfires, especially if it affects fuel injectors.

Understanding the cause of an engine misfire allows targeted action—replacing spark plugs, ignition coils, or cleaning airflow components. By following these steps and using tools from O’Reilly Auto Parts, you can restore performance, prevent further engine issues, and ensure reliable operation.