P0301 Code: Cylinder 1 Misfire Detected
P0301 means the engine computer detected a misfire in cylinder 1. The fastest way to pin it down is the coil-swap test — most P0301s come down to a spark plug, ignition coil, or injector on that one cylinder.
⚡What Does P0301 Mean?
| Code | P0301 |
|---|---|
| Definition | Cylinder 1 Misfire Detected |
| System | Combustion (Ignition + Fuel + Air) |
| Severity | ⛔ Moderate–High — an active misfire can overheat and damage the catalytic converter |
| Common Related Codes | P0300 (random/multiple misfire), P0302–P0308 (other cylinders), P0171 (lean), P0101 (MAF) |
A misfire means cylinder 1 isn’t producing consistent power. The PCM detects it by watching for tiny changes in crankshaft speed each time that cylinder should fire. The root cause is usually something that interrupts proper combustion: not enough spark, not enough fuel, the wrong amount of air, or a mechanical problem that reduces compression.
Because the code names a specific cylinder, P0301 is often quicker to diagnose than a random misfire — the fault is isolated to cylinder 1’s plug, coil, injector, or that cylinder’s mechanical condition.
🔎Symptoms of P0301
- Rough idle or shaking at idle
- Hesitation or stumble during acceleration
- Loss of power and poor acceleration
- Hard starting
- A fuel smell from the exhaust (sometimes)
- Check Engine Light on — may flash if the misfire is severe
🔧Common Causes of P0301
Ignition (most common)
- Worn or fouled spark plug on cylinder 1
- Weak or failing ignition coil (coil-on-plug or coil pack)
- Damaged plug wire, if equipped
- Oil or moisture in the spark plug well
Fuel delivery
- Clogged or failing fuel injector on cylinder 1
- Injector wiring or connector problems
- Low fuel pressure (can still flag a cylinder-specific code)
Air / vacuum leaks
- Vacuum leak near the cylinder 1 intake runner
- Intake manifold gasket leak
- PCV system leaks — often sets a lean code like P0171 too
Mechanical
- Low compression on cylinder 1
- Burnt or sticking valve
- Timing problems
- Head gasket issues (less common, but possible)
🚦How Serious Is P0301?
P0301 is moderate to high severity. A minor, intermittent misfire may feel small at first, but it tends to worsen — and a continuous misfire dumps unburned fuel into the exhaust, which can overheat and destroy the catalytic converter. If drivability is poor or the Check Engine Light is flashing, treat it as urgent and address it promptly.
🩺How to Diagnose P0301
- Scan for additional codes — lean codes, fuel-trim codes, or other misfire codes help point to a root cause beyond a single bad part.
- Do the coil-swap test. Move the ignition coil from cylinder 1 to a known-good cylinder. If the misfire follows the coil (the code changes cylinders), the coil is bad. If the misfire stays on cylinder 1, the problem is the plug, injector, or compression on that cylinder.
- Inspect the cylinder 1 spark plug for wear, cracks, oil fouling, or an incorrect gap. A swap with a known-good plug works the same way as the coil swap.
- Check the injector for cylinder 1 — listen for consistent clicking and confirm the connector is secure.
- Review fuel trims to see if a lean condition is contributing.
- Inspect for vacuum leaks near the intake manifold and hoses.
- Compression or leak-down test if ignition and fuel checks don’t resolve the misfire — this catches mechanical causes.
Misfire on Cylinder 1 — Coil, Plug, or Injector?
Describe your vehicle and symptoms to our free AI Diagnostic — it ranks the most likely cause of your P0301 and tells you what to check first.
🛠️How to Fix P0301
| Repair | Most likely when |
|---|---|
| Replace spark plug (cyl 1) | Plug is worn, fouled, cracked, or gapped wrong |
| Replace ignition coil (cyl 1) | Misfire follows the coil in a swap test |
| Fix oil / moisture intrusion | Oil in the plug well or water causing an intermittent misfire |
| Service or replace fuel injector (cyl 1) | Injector is restricted/failing or its connector is faulty |
| Repair vacuum leak / intake gasket | Lean trims or a hiss/leak near the intake |
| Mechanical repair | Compression is low or there’s a valve/timing issue |
💰P0301 Repair Cost Estimates
| Repair | Estimated Cost |
|---|---|
| Spark plug replacement | $30 – $150 |
| Ignition coil replacement | $80 – $350 |
| Fuel injector cleaning | $80 – $200 |
| Fuel injector replacement | $150 – $600 |
| Vacuum leak repair | $50 – $250 |
| Compression / leak-down testing | $120 – $300 |
Costs vary by make, model, and engine access. On most vehicles, a plug or coil is the cheapest and most common fix — which is why the coil-swap test is worth doing before buying anything.