The engine shudders at idle. Acceleration feels rough, and the check engine light flashes when you climb a hill. A code reader shows P0302: cylinder 2 misfire detected. A misfire means one or more cylinders are not burning fuel properly. Finding out why is usually a process of elimination.
Key Takeaways
- Combustion needs spark, the right fuel mixture and good compression; a misfire means one of these is missing.
- A single-cylinder misfire often points to a component on that cylinder, such as a spark plug, coil or injector.
- The coil swap test can show whether a misfire follows a coil to a different cylinder.
- A flashing check engine light indicates a severe misfire; reduce load and get it fixed promptly to avoid catalytic converter damage.
What a misfire feels like
- Rough idle or shaking
- Hesitation or stumbling during acceleration
- Loss of power
- Poor fuel economy
- Check engine light, possibly flashing
- Smell of fuel from the exhaust
The three ingredients of combustion
Each cylinder needs:
- Spark at the right time, from the ignition system
- Fuel and air in the correct proportion
- Compression — the cylinder must seal properly
A problem with any of these can cause a misfire.
Ignition causes
Spark plugs. Worn electrodes increase the voltage needed to create a spark. Fouled plugs, covered in oil or carbon, may not spark reliably. Many modern iridium or platinum plugs last a long time, but intervals vary. Follow your maintenance schedule.
Ignition coils. Most modern engines use a coil-on-plug design, with one coil per cylinder. Coils can fail from heat and age, often causing misfires under load or in damp conditions.
Spark plug wires. On older engines, damaged wires can leak spark.
Fuel causes
Fuel injectors. A clogged or failing injector can deliver too little fuel to one cylinder. A leaking injector can deliver too much.
Fuel pressure. Low pressure from a weak pump, clogged filter or faulty regulator usually affects all cylinders, causing random misfires (P0300).
Vacuum leaks. Extra unmetered air can make the mixture lean, especially at idle. A leak near one cylinder’s intake runner can cause a single-cylinder misfire.
Compression and mechanical causes
Low compression. Worn piston rings, burned valves or a failed head gasket can prevent a cylinder from sealing. A compression test or leak-down test can identify these problems.
Valve timing problems. A stretched timing chain or jumped belt can affect multiple cylinders.
The coil swap test
For engines with coil-on-plug ignition, a simple test can help:
- Note which cylinder is misfiring (for example, cylinder 2).
- With the engine off, swap the coil from cylinder 2 with a coil from another cylinder (for example, cylinder 3).
- Clear the code and drive under conditions that caused the misfire.
- If the misfire moves to cylinder 3, the coil is likely faulty. If it stays on cylinder 2, look at the plug, injector or compression.
The same approach can be used with spark plugs, and sometimes injectors, though injector swaps are more involved.
Reading the spark plug
Removing a spark plug can reveal clues:
- Oily deposits: oil entering the cylinder
- Black, sooty deposits: rich mixture or weak spark
- White, blistered insulator: overheating or lean mixture
- Worn electrode gap: plug past its service life
Allow the engine to cool before removing plugs, and use the correct torque when installing new ones.
Is it safe to keep driving?
A mild intermittent misfire may allow short-term driving, but a misfire should be fixed promptly:
- Unburned fuel can overheat and damage the catalytic converter.
- Engine performance and fuel economy suffer.
- A flashing check engine light indicates a severe misfire.
If the light is flashing, reduce speed and load, avoid hard acceleration, and get the car checked as soon as possible.
Frequently Asked Questions
Can bad fuel cause misfires? Contaminated fuel can cause misfires, usually across multiple cylinders.
Should I replace all spark plugs at once? Yes, plugs are generally replaced as a set.
Do I need to replace all coils if one fails? Not necessarily. Some owners replace all coils on high-mileage engines to prevent repeat failures, but it is not required.
Can a misfire be caused by a sensor? Faulty sensors affecting fuel mixture or timing can contribute to misfires.
The bottom line
A misfire is a symptom. Work through spark, fuel and compression, use the coil swap test to narrow down the cause, and fix it quickly to protect your catalytic converter.