P0301 means the engine has detected misfires specifically in cylinder 1 — and that cylinder-specific detail is genuinely good news for diagnosis. Unlike a random misfire, you know exactly which cylinder to look at, and the swap test — moving the coil or plug to another cylinder and watching whether the code follows — identifies the faulty part without buying anything. If the light is flashing, stop driving first.
What Does P0301 Mean?
P0301 is Cylinder 1 Misfire Detected. The ECU watches tiny variations in crankshaft speed — every properly firing cylinder gives the crank a small acceleration, and when cylinder 1’s expected contribution repeatedly fails to arrive, this code is set.
The last two digits identify the cylinder: P0301 is cylinder 1, P0302 is cylinder 2, and so on. This is the key difference from P0300, which means misfires spread across multiple cylinders and points at a shared cause. A single-cylinder code points at the parts belonging to that one cylinder — its coil, its plug, its injector, or the cylinder itself.
It varies by engine. On most inline engines it is the cylinder nearest the crankshaft pulley (the belt end of the engine), but on V engines and some transverse layouts it differs by manufacturer. Check a reference for your specific engine before pulling anything apart — working on the wrong cylinder is a surprisingly common way to lose an afternoon.
Common Symptoms
- Rough idle — the engine shakes noticeably at a standstill, since one cylinder in four, five or six is doing nothing.
- A flashing or steady check engine light. Flashing means the misfire is active and severe — see check engine light flashing before driving further.
- Loss of power and hesitation, most noticeable under load.
- Worse fuel economy, since unburned fuel from the dead cylinder is wasted.
- A smell of unburned fuel from the exhaust.
- Vibration felt through the cabin — see car shakes at idle for how a single-cylinder misfire compares to other causes of idle shake.
Common Causes
Listed in the order they turn out to be responsible:
- A failed ignition coil. On modern coil-on-plug engines, the single most common cause of a cylinder-specific misfire. One coil dies; its cylinder stops firing.
- A worn or fouled spark plug. The plug in that cylinder may be worn beyond its gap limit, cracked, or fouled with oil or fuel deposits.
- A failed or clogged fuel injector. An injector that has failed electrically or is blocked delivers no fuel — or the wrong amount — to that one cylinder.
- A vacuum leak affecting one cylinder — a leaking intake manifold gasket at one runner can lean out a single cylinder while the others run normally.
- Low compression in that cylinder. A burnt exhaust valve, a damaged piston ring, or a head gasket failure at that cylinder. This is the expensive end, and a compression test settles it.
- Wiring or connector damage on that cylinder’s coil or injector circuit.
The Swap Test — Diagnosis for Free
Because the code names one cylinder, you can identify the faulty component without buying anything, using parts the engine already has.
1. Swap the ignition coil
Move the coil from cylinder 1 to another cylinder (say, cylinder 3), and fit that cylinder’s coil in position 1. Clear the code and run the engine.
- Code moves to P0303 — the coil is faulty. Replace it.
- Code stays at P0301 — the coil is fine; move to the plug.
2. Swap the spark plug the same way
If the code follows the plug, the plug is the fault. If it stays, keep going.
3. Then the injector
Same principle, though injector access is harder on many engines — at this point a workshop may be the practical choice.
4. If nothing follows: compression test
A code that stays on cylinder 1 through coil, plug and injector swaps points at the cylinder itself — compression testing tells you whether the problem is mechanical.
Ignition coils generate tens of thousands of volts. Switch the ignition off and disconnect the battery negative terminal before removing coils or plugs, and let the engine cool first — plugs sit deep in a hot cylinder head.
Possible Fixes
| Cause | Fix | Typical cost |
|---|---|---|
| Failed ignition coil | Replace the coil | $50 – $150 |
| Worn or fouled spark plug | Replace the plug (ideally the full set) | $20 – $250 |
| Failed fuel injector | Replace or professionally clean | $150 – $400 |
| Intake gasket leak at one runner | Replace the gasket | $200 – $600 |
| Wiring fault | Repair the circuit | $80 – $250 |
| Low compression (valve, ring, gasket) | Internal engine repair | $600 – $3,000+ |
If one coil has failed from age, its siblings are the same age — many owners replace coils as a set once the second one goes. Plugs should almost always be replaced as a complete set regardless.
Can I Drive With P0301?
Only if the check engine light is steady, and only gently for a short distance. A flashing light means raw fuel from the dead cylinder is reaching the catalytic converter and can destroy it within minutes — that is a stop-driving situation, covered fully in our flashing check engine light guide.
A sustained misfire also washes oil from that cylinder’s walls and dilutes the sump oil — this is not a fault to live with for weeks. If the misfire has already been present a while, keep an eye out for P0420 appearing afterwards, a common sign the converter took damage.
When to See a Mechanic
Hand it over if:
- The light is flashing and you are not equipped to diagnose it immediately.
- The swap tests point past the coil, plug and injector to the cylinder itself.
- The misfire is intermittent and refuses to reproduce on demand.
- A compression test shows cylinder 1 significantly below the others.
Mention which swaps you have already done — that alone can save a workshop an hour of repeating the same steps. More in our trouble codes section.
Frequently Asked Questions
What is the most common cause of P0301?
A failed ignition coil on that cylinder, followed by a worn or fouled spark plug. Between them they account for the large majority of cylinder-specific misfire codes on modern coil-on-plug engines.
How is P0301 different from P0300?
P0301 names cylinder 1 specifically, which points at that cylinder’s own components — coil, plug, injector. P0300 means misfires across multiple cylinders and points at something shared, like a vacuum leak or fuel delivery.
Can I drive with a P0301 code?
Briefly and gently if the light is steady. Not at all if it is flashing — an active misfire dumps raw fuel into the catalytic converter and can destroy it within minutes of driving.
How do I know if it’s the coil or the spark plug?
Swap the suspect coil with one from another cylinder, clear the code, and run the engine. If the code moves to the other cylinder, the coil is faulty. If it stays, repeat the test with the plug.
How much does it cost to fix P0301?
Often $50 – $250 for a coil or plugs, which covers most cases. If compression testing reveals a mechanical fault in the cylinder, costs rise to $600 – $3,000 or more.
