You can drive with a misfire, but not safely or for long

A misfire means one or more of your engine's cylinders is not firing properly — the spark plug ignites the fuel-air mixture late, not at all, or incompletely. You can physically operate the vehicle and move it down the road, but the engine is not running at full power, fuel economy drops sharply, and continued driving causes damage to the catalytic converter and other emissions equipment. Most mechanics recommend stopping and diagnosing the problem before driving more than a few miles.

The risk is not that your car will suddenly stop. The risk is that you are burning unburned fuel in the exhaust system, which heats the catalytic converter beyond its design temperature. A catalytic converter that overheats can fail completely, and replacing one costs between $800 and $2,500 depending on the vehicle. Driving on a misfire for weeks or months is the most common way owners end up with that bill.

Key Takeaways

  • A misfire allows you to drive but damages the catalytic converter if you continue, which is an expensive repair.
  • The check engine light comes on during a misfire because the engine computer detects that a cylinder is not firing on schedule.
  • Common causes are a bad spark plug, a fuel injector that is clogged or stuck, a coil pack failure, or a compression problem in that cylinder.
  • You should have the vehicle scanned for a diagnostic trouble code within a day or two, because the code tells you which cylinder is misfiring and narrows the cause.
  • Continuing to drive on a misfire for more than a few days risks permanent damage to the catalytic converter and oxygen sensors.

Why the check engine light turns on during a misfire

Your engine computer monitors the firing of each cylinder by watching the oxygen sensor in the exhaust. When a cylinder misfires, unburned fuel enters the exhaust, and the oxygen sensor detects a change in the exhaust composition. The computer logs a diagnostic trouble code — usually P0300 (random misfire), P0301 (cylinder 1 misfire), P0302 (cylinder 2 misfire), and so on — and illuminates the check engine light.

The light itself is not a warning that your engine will fail when ready. It is a signal that the computer has detected a problem and stored a code. You can read that code with a scanner (many auto parts stores scan for free), and the code tells you which cylinder is misfiring. That information is the first step toward finding the cause.

Common reasons a single cylinder misfires

A misfire in one cylinder usually points to one of four problems: a worn or fouled spark plug, a fuel injector that is not spraying fuel properly, a coil pack or ignition coil that has failed, or low compression in that cylinder due to a valve or piston problem.

Spark plugs wear out over time — most last 30,000 to 100,000 miles depending on the type — and a plug that is worn or covered in carbon will not fire reliably. Fuel injectors can become clogged from carbon buildup or stuck in the open or closed position, preventing the right amount of fuel from entering the cylinder. Coil packs sit on top of spark plugs and generate the high voltage needed to ignite the fuel; a coil pack that fails will not fire that plug. Compression problems are less common in newer cars but more serious: they mean a valve is not sealing, a piston ring is worn, or the head gasket is leaking, and they require engine work to fix.

A mechanic will usually start by checking the spark plug and coil pack for that cylinder, then move to fuel pressure and injector spray pattern if those are not the problem. Compression testing comes later if the simpler causes are ruled out.

What happens if you keep driving on a misfire

The when ready effect is poor performance: the engine runs rough, acceleration is weak, and fuel economy drops noticeably. You may feel a vibration or hear a popping sound from the exhaust. The long-term effect is damage to the catalytic converter.

The catalytic converter is designed to burn off unburned fuel in the exhaust at a controlled temperature. When a cylinder misfires, raw fuel enters the exhaust continuously, and the converter tries to burn it all at once. This causes the converter to overheat. After days or weeks of this, the internal structure of the converter breaks down, and it stops working. At that point, the check engine light stays on, emissions testing fails, and you have a $1,000+ repair bill.

Oxygen sensors can also be damaged by prolonged exposure to a misfire, and they are another expensive component to replace. The longer you drive on a misfire, the more systems you risk damaging.

When a misfire affects multiple cylinders

If the diagnostic code is P0300 (random misfire) or if multiple cylinders are misfiring, the cause is usually not a single spark plug or coil. Common culprits are low fuel pressure (a failing fuel pump or clogged fuel filter), a vacuum leak that leans out the fuel mixture, bad gas with water or contaminants in it, or a problem with the ignition system that affects all cylinders equally.

Multiple misfires are also more serious because they mean the engine is not running on most of its cylinders. Performance is severely degraded, and the catalytic converter is being damaged much faster. If you see a code for multiple misfires, do not drive the vehicle except to a repair shop.

Steps to take when you notice a misfire

First, note what the engine feels like: does it run rough all the time, or only when accelerating or at idle? Does it happen when the engine is cold, or after it warms up? Does it happen after you fill up with gas? These details help a mechanic narrow down the cause.

Second, have the vehicle scanned for a diagnostic trouble code as soon as you can — within a day or two. The code tells you which cylinder or cylinders are misfiring, and that is the most important piece of information you can give a mechanic. Many auto parts stores offer free scanning.

Third, limit your driving. A few miles to a repair shop is acceptable. Driving hundreds of miles on a misfire is not. If you cannot reach a shop quickly, park the vehicle and arrange a tow or wait until you can take it in.

Fourth, do not ignore the problem hoping it will go away. Misfires do not self-correct. The underlying cause — a bad plug, a failed coil, a fuel system problem — will not fix itself, and the longer you wait, the more damage accumulates.

Misfire versus other engine problems

A misfire is not the same as a rough idle, though the two can feel similar. A rough idle means the engine shakes or vibrates when stopped or at very low speed, but runs smoothly once you accelerate. A misfire is felt at all speeds and is usually accompanied by a loss of power.

A misfire is also not the same as a knock or ping, which is a sharp metallic sound from the engine when you accelerate hard. A knock is usually caused by low-octane fuel or carbon buildup and is a different problem with different solutions.

The check engine light can come on for dozens of reasons, but if the light is on and the engine feels rough or shaky, a misfire is a likely cause. The diagnostic code will confirm it.

Frequently Asked Questions

Can I drive to the mechanic if my check engine light is on and the engine feels rough?

Yes, if the shop is within a few miles. Driving a short distance on a misfire is acceptable. Driving long distances — more than 20 or 30 miles — risks serious damage to the catalytic converter. If the shop is far away, call ahead and ask if they can fit you in quickly, or arrange a tow.

What does a misfire sound like?

A misfire usually sounds like a popping or spitting noise from the exhaust, especially when you accelerate. You may also hear a rough, uneven engine sound, like the engine is skipping. The sound is most noticeable when the engine is under load.

Will a misfire cause my car to fail an emissions test?

Yes. A misfire causes the check engine light to come on and stores a diagnostic code, and most emissions tests will fail if the check engine light is illuminated. You must have the misfire repaired and the code cleared before the vehicle will pass.

Is a misfire always expensive to fix?

Not always. A worn spark plug costs $10 to $50 and takes 15 minutes to replace. A failed coil pack costs $50 to $200. A fuel injector cleaning or replacement can cost $300 to $900. A catalytic converter replacement costs $800 to $2,500. The cost depends on the cause, so the diagnostic code is essential — it tells you what you are actually paying for.

Can I drive with a misfire if I am careful and do not accelerate hard?

Driving carefully does not prevent damage to the catalytic converter. The converter is damaged by heat, and that heat builds up whether you accelerate hard or drive gently. The only way to prevent converter damage is to have the misfire repaired. Careful driving just delays the damage a few extra days.