Misfire · Engine code
P0301 and P0302–P0308: Misfire Detected in One Cylinder
One cylinder — the number in the last digit of the code — isn't burning its fuel properly. P0301 is cylinder 1, P0302 is cylinder 2, and so on up to P0308.
What this code means
The engine computer watches crankshaft speed and can tell exactly which cylinder stumbled. When the misfires stay on one cylinder, it sets a code ending in that cylinder's number: P0301 for cylinder 1 through P0308 for cylinder 8.
That's good news for diagnosis. A problem limited to one cylinder almost always comes down to a part that only serves that cylinder: its spark plug, its ignition coil, its fuel injector, or the valves and rings in that cylinder.
Cylinder numbering depends on the engine. On most inline four-cylinder engines, cylinder 1 is at the end with the drive belts. On V6 and V8 engines the numbering differs by manufacturer, so check a diagram for your exact engine before you start swapping parts.
Symptoms you may notice
- A steady, rhythmic shake at idle
- Rough running that may smooth out at higher speed
- Check engine light, sometimes flashing when you accelerate
- Reduced power and fuel economy
Common causes, most likely first
- Failed ignition coilThe most common cause on modern engines, where every cylinder has its own coil sitting on top of the spark plug.
- Worn or damaged spark plugA cracked insulator, oil-fouled plug or worn electrode on that one cylinder.
- Oil in the spark plug wellA leaking valve cover or tube seal can let oil pool around the plug and short out the spark.
- Clogged or leaking fuel injectorThe cylinder gets too little fuel, or too much.
- Vacuum leak at that cylinder's intake runnerA leaking intake gasket next to one cylinder can lean out just that cylinder.
- Low compression in that cylinderA burned valve, worn rings or head gasket leak. Less common, but it's the cause when the coil, plug and injector all check out.
What to check first
- Swap the coil. Move the ignition coil from the misfiring cylinder to a neighboring one, clear the code, and drive. If the misfire follows the coil (P0301 becomes P0302, for example), the coil is bad. This test costs nothing and is the single most useful step.
- If the misfire stays put, pull that cylinder's spark plug and compare it with the others. Look for cracks, heavy carbon, oil or a worn electrode.
- Check the plug well for oil. If there's oil, the valve cover gasket or tube seals need replacing too, or the new plug and coil will fail again.
- If coil and plug are fine, an injector test or swap (where practical) is next.
- If everything external checks out, a compression test on that cylinder confirms or rules out an internal problem.
Typical repairs and rough cost
| Repair | Typical cost | Notes |
|---|---|---|
| Ignition coil (one cylinder) | $100–$350 | Many owners replace this themselves on four-cylinder engines. |
| Spark plug(s) | $100–$350 | If plugs are due, replace all of them, not just the one. |
| Valve cover gasket / tube seals | $150–$500 | Needed when oil is in the plug wells. |
| Fuel injector | $200–$600 | Varies a lot between port and direct injection. |
Rough US shop prices for parts and labor. Your cost depends on the vehicle, the parts brand and local labor rates.
Official code definitions
| Code | Applies to | Definition |
|---|---|---|
| P0301 | Generic | Cylinder 1 Misfire Detected |
| P0302 | Generic | Cylinder 2 Misfire Detected |
| P0303 | Generic | Cylinder 3 Misfire Detected |
| P0304 | Generic | Cylinder 4 Misfire Detected |
| P0305 | Generic | Cylinder 5 Misfire Detected |
| P0306 | Generic | Cylinder 6 Misfire Detected |
| P0307 | Generic | Cylinder 7 Misfire Detected |
| P0308 | Generic | Cylinder 8 Misfire Detected |
“Generic” codes mean the same thing on every make. Manufacturer entries are that brand’s own description. Look up any other code in the code library.
Common questions
Which cylinder is cylinder 1?
On most inline four-cylinder engines, it's the one closest to the accessory belts. On V6 and V8 engines it depends on the manufacturer. Search for a cylinder diagram for your exact engine.
Should I replace all the coils at once?
Usually not. Coils fail individually, and the swap test tells you which one is bad. Spark plugs are different: if they're near their service interval, replace the full set.
Can I keep driving with a misfire in one cylinder?
Gently and for a short time, if the light is steady. A flashing light means stop driving when you safely can. Unburned fuel overheats the catalytic converter, which is a much bigger repair.
Related codes
This guide explains what the code usually means and how it’s typically diagnosed. Every car is different, and a code points to a system, not always a specific part. For anything affecting brakes, steering or a car that feels unsafe, have a qualified mechanic confirm before you rely on it.