# What is an engine misfire? ## Quick answer A misfire happens when one or more cylinders do not burn their air and fuel properly, so the engine loses power and runs roughly. ## What is Misfire? An engine makes power by burning a mix of air and fuel in each cylinder, lit by a spark plug. If that burn does not happen properly in a cylinder, it is called a misfire. You may feel shaking, a rough idle or a stumble when accelerating. Misfires can be constant, or show up only under load, when cold or in damp weather. Common causes are worn spark plugs, a bad ignition coil, a clogged fuel injector, a vacuum leak or low compression. The car's computer detects misfires and sets codes such as P0300 for random misfires or P0301 to P0306 for a specific cylinder. The last digit tells you the cylinder number, which helps a mechanic start in the right place. A common shop test is to swap the ignition coil or spark plug from the misfiring cylinder to another one. If the misfire code moves to the new cylinder, the swapped part is the problem. If it stays, the cause is more likely fuel delivery, a vacuum leak or a mechanical problem such as low compression. A flashing check engine light usually means a misfire bad enough to damage the catalytic converter. Ease off the gas and get the car checked as soon as possible. Unburned fuel from a misfiring cylinder overheats the converter, turning a cheap plug or coil repair into a much costlier one. ## How it works 1. Each cylinder needs air, fuel and a spark at the right time to burn properly. 2. If any of these is missing or weak, that cylinder does not fire properly. 3. The computer watches crankshaft speed and notices the brief slowdown a misfire causes. 4. It stores a misfire code and flashes the check engine light if the misfire is severe. ## Types of Misfire | Type | What it is | | --- | --- | | Ignition misfire | No spark or a weak spark, usually from worn spark plugs, a failing ignition coil or damaged plug wires. | | Fuel misfire | Too little or too much fuel in a cylinder, from a clogged or leaking injector or low fuel pressure. | | Mechanical misfire | Low compression from a burnt valve, worn rings, a head gasket leak or a timing problem. | | Random versus single-cylinder | P0300 means misfires across several cylinders, often a shared cause, while P0301 to P0312 point to one cylinder. | ## Common Misfire problems - Worn or fouled spark plugs, especially past the replacement interval. - A failing ignition coil, often shown by a misfire code that follows the coil when swapped. - A clogged or leaking fuel injector on one cylinder. - A vacuum leak causing lean misfires, often with P0171 or P0174. - Low compression from valve, ring or head gasket problems. ## Frequently asked questions ### Is it safe to drive with an engine misfire? A light, occasional misfire may be OK for a short drive to a shop. If the check engine light is flashing or the car shakes badly, stop driving or drive gently only as far as you must, because a severe misfire can damage the catalytic converter and make the car hard to control in traffic. ### What is the most common cause of a misfire? Worn spark plugs and failing ignition coils are among the most common causes. Fuel injector problems and vacuum leaks are also common. Less often, the cause is mechanical, such as low compression from a burnt valve or a head gasket leak, which a compression test can confirm. ### What does P0300 mean? P0300 means the computer detected random misfires in more than one cylinder. Codes P0301 to P0306 point to a specific cylinder, and larger engines go higher. P0300 often has a shared cause, such as a vacuum leak, low fuel pressure or worn plugs across the whole set. ### What does a misfire feel like? A misfire often feels like a shake, shudder or stumble, especially at idle or when accelerating. The engine may sound uneven, and the car may feel down on power. Some light misfires are hard to feel at all and only show up as a check engine light. ### How much does it cost to fix a misfire? It depends on the cause. Spark plug replacement at an independent shop often costs about $120 to $450, and one ignition coil about $150 to $400. Injector, vacuum leak or engine repairs vary more. Diagnosis first avoids paying for parts that are not the problem. ### Can bad gas cause a misfire? It can. Fuel contaminated with water or debris, or fuel with the wrong octane for an engine that requires premium, can cause rough running and misfires. If misfires start right after filling up, fuel quality is a possibility. Persistent misfires usually need a proper diagnosis. ### Can a misfire fix itself? Sometimes a misfire caused by damp weather or a brief fuel issue goes away, and the light may turn off after several drives. Most misfires come from wear, such as plugs or coils, and get worse over time. Have the codes read even if the problem seems to stop. ### Why does my engine misfire only when cold? Cold misfires can come from worn plugs, a weak coil, carbon buildup on intake valves in direct-injection engines, or a leaking injector. Some clear once the engine warms up. Note when it happens and share that with your mechanic, along with any stored codes. ### Will a misfire damage my engine? A misfire usually damages the catalytic converter before the engine. A cylinder that misfires constantly can wash oil off the cylinder wall with unburned fuel, adding wear over time. Fix misfires promptly to protect both the converter and the engine. ### Can I find which cylinder is misfiring myself? A code reader shows the cylinder for codes like P0301 to P0306. Your repair manual shows which cylinder is number 1. You can swap that cylinder's coil with a neighbor's, clear the code and drive. If the code moves, the coil is the likely problem. ## Sources - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Related terms - [Check engine light](https://diagxcar.com/glossary/check-engine-light) - [DTC](https://diagxcar.com/glossary/dtc) - [Catalytic converter](https://diagxcar.com/glossary/catalytic-converter) - [Fuel trim](https://diagxcar.com/glossary/fuel-trim) ## Related pages - [P0300 Random/Multiple Cylinder Misfire Detected](https://diagxcar.com/codes/p0300) - [P0301 Cylinder 1 Misfire Detected](https://diagxcar.com/codes/p0301) - [Spark plug replacement cost](https://diagxcar.com/cost/spark-plug-replacement): $120-$450 - [Ignition coil replacement cost](https://diagxcar.com/cost/ignition-coil-replacement): $150-$400 - [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle) --- Source page: https://diagxcar.com/glossary/misfire Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.