P0301: Cylinder 1 Misfire Detected
P0301 means the engine computer detected a misfire specifically in cylinder number one. Unlike P0300, the fault is isolated to a single cylinder, which narrows diagnosis considerably: the cause is almost always that cylinder's spark, fuel or compression.
Can I still drive with P0301?
No, if the light is flashing
A flashing light means raw fuel is reaching the catalytic converter and can ruin it quickly. Stop and arrange a tow. A steady light with mild roughness allows a short, gentle drive to a shop.
What you will actually notice
- Noticeable vibration or shaking, worst at idle
- Loss of power, particularly under acceleration
- Check engine light, often flashing
- Rough, uneven engine note
- Higher fuel consumption
Causes, ordered by how often they turn out to be the fault
1.Faulty ignition coil on cylinder 1
Very commonEasily confirmed by swapping the coil with a neighbouring cylinder.
2.Worn or fouled spark plug in cylinder 1
Very commonPull it and read it — its condition often reveals the underlying cause.
3.Clogged or failing injector on cylinder 1
CommonSuspect this when spark components test good.
4.Vacuum leak local to that cylinder
Less commonA cracked intake runner gasket can affect a single cylinder.
5.Low compression in cylinder 1
Less commonBurnt valve, worn rings or head gasket. Confirm before acting.
6.Damaged wiring to the coil or injector
RareCheck the connector before condemning parts.
How to diagnose it, cheapest test first
Work down this list in order. The most expensive mistake with this code is replacing a costly part before ruling out a cheap one.
- 1
Identify which physical cylinder is number one on your engine
FreeIt is not always the one nearest the front. Getting this wrong wastes the entire diagnosis.
- 2
Swap the coil from cylinder 1 with a neighbouring cylinder, clear codes, drive
FreeIf the code moves to the other cylinder, the coil is the fault. This costs nothing and settles it.
- 3
Inspect and replace the spark plug
LowOil on the plug points at valve seals; coolant points at a head gasket.
- 4
Swap the injector the same way
Low to moderateSame logic as the coil test.
- 5
Compression test cylinder 1
ModerateOnly after spark and fuel have been eliminated.
What the repairs typically cost
| Repair | Estimated range |
|---|---|
| Ignition coil replacementOne coil plus labour. | $120 – $500 |
| Spark plug replacementUsually done as a full set. | $80 – $350 |
| Fuel injector replacementPer injector on most engines. | $200 – $700 |
| Head gasket or valve repairThe worst case, and why compression testing comes last. | $1,000 – $3,500 |
Estimate only. Ranges vary widely by region, vehicle and shop rate, and are not quotes. Always get a written estimate before authorising work.
Notes for specific makes
Subaru
On flat engines, cylinder numbering is not left to right. Verify against service data before pulling anything.
Volkswagen / Audi
Single-coil failure is common and coils are often replaced as a set to avoid repeat visits.
Toyota
Check for oil in the spark plug tube, a known cause of single-cylinder misfire on several engines.
Common questions
Which cylinder is number one?
It varies by engine and is not reliably the one closest to the front of the car. Check your service data — swapping the wrong coil tells you nothing.
Is it safe to swap coils to test?
Yes, and it is the single most useful free test for this code. If the misfire follows the coil to its new cylinder, you have your answer.
Why did the code come back after new plugs?
Because plugs were a symptom, not the cause. Oil fouling points at valve stem seals; fuel fouling points at an injector.
Not sure which of these applies to your car?
Describe what the car is actually doing and get a diagnosis narrowed to your vehicle, or find a shop near you.
AI-generated and informational only. This is not a diagnosis of your specific vehicle and does not replace inspection by a qualified technician.