P0301 — Cylinder 1 Misfire Detected
SevereQuick answer
P0301 means the engine computer has detected that cylinder 1 is misfiring — its air-fuel mixture isn't burning properly. The most common causes are a worn spark plug or a failing ignition coil. If your check engine light is flashing, stop driving: continued misfires can destroy the catalytic converter.
What it means
Your engine's computer (the ECM) constantly watches how fast the crankshaft accelerates after each cylinder fires. When cylinder 1 doesn't contribute the power pulse it should — because the mixture didn't ignite, ignited weakly, or ignited at the wrong time — the ECM counts it as a misfire. Enough misfires in a short window and it stores P0301 and turns on the check engine light.
The "1" in P0301 refers to the cylinder number, not the firing order position. Cylinder 1's location varies by engine — it's marked in your repair manual, and our VIN tool can point you to the right service info for your exact vehicle.
A misfiring cylinder dumps unburned fuel into the exhaust. The catalytic converter tries to burn that fuel and overheats — which is why a steady light means "fix it soon" but a flashing light means "stop now." A flashing check engine light is the ECM telling you converter damage is actively happening.
Common causes
Ordered from most to least likely.
- 1.
Worn or fouled spark plug
By far the most common cause, especially past the plug's service interval. Oil or carbon deposits on the tip prevent a clean spark.
- 2.
Failing ignition coil
On modern coil-on-plug engines, each cylinder has its own coil. A cracked or internally shorted coil weakens or kills the spark. Coils often fail when hot, so the misfire may come and go.
- 3.
Damaged spark plug wire or boot
Older engines with plug wires: cracked insulation lets the spark leak to ground, especially in damp weather.
- 4.
Clogged or leaking fuel injector
A dirty injector starves the cylinder of fuel; a leaking one floods it. Either way the mixture won't burn right.
- 5.
Vacuum leak near cylinder 1
A torn intake gasket or cracked vacuum hose leaning out just that cylinder's mixture.
- 6.
Low compression
A burnt exhaust valve, worn rings, or a failing head gasket. More likely on high-mileage engines or if the misfire is constant and the cheap fixes haven't helped.
- 7.
Wiring or connector damage at the coil/injector
Chafed harness, corroded connector pins, or rodent damage interrupting power to that cylinder's coil or injector.
How to diagnose it, step by step
Cheapest and most likely checks first.
-
1 Scan and read the freeze frame
Read the codes with an OBD-II scanner and note the freeze frame data: engine temperature, RPM, and load when the misfire happened. If P0301 appears together with lean codes (P0171) or cam/crank sensor codes, chase those first — they can cause misfires as a side effect.
-
2 Inspect spark plug 1
Pull the cylinder 1 spark plug and read it. Oily, sooty, cracked porcelain, or a gap worn far past spec all point to the plug. If the plugs are past their service interval, replace the full set, not just one — the others are right behind it.
-
3 Swap the ignition coil to another cylinder
Move cylinder 1's coil to another cylinder (say cylinder 3) and clear the codes. If the misfire follows the coil — the code comes back as P0303 — the coil is your problem. This costs nothing and is the single most useful misfire test.
-
4 Check the injector
Listen for a steady clicking at the injector with a long screwdriver or mechanic's stethoscope while the engine idles. Silent injector means no pulse (wiring) or a stuck injector. You can swap injectors between cylinders the same way you swapped coils.
-
5 Look for vacuum leaks
Inspect hoses and the intake gasket area near cylinder 1. A spray of carb cleaner around suspected spots that changes the idle reveals the leak. Smoke testing is the thorough version if you have access to a smoke machine.
-
6 Run a compression or leak-down test
If spark, fuel, and vacuum all check out, measure compression on cylinder 1 and compare with the others. More than ~10–15% below its neighbors points to a mechanical problem — valves, rings, or head gasket — and that's a bigger job worth confirming with a leak-down test.
Parts & tools you may need
- OBD-II scanner (code reader with freeze frame)
- Spark plug socket set with extension
- Replacement spark plugs (full set, correct part for your engine)
- Ignition coil (cylinder-specific, if the swap test confirms it)
- Torque wrench (spark plugs have a torque spec — don't guess)
- Dielectric grease for coil boots
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Related codes
Frequently asked questions
- Can I drive with P0301?
- Short distances with a steady check engine light, yes — gently. If the light is flashing, no: a flashing light means raw fuel is overheating the catalytic converter, and converters cost far more than any likely fix for the misfire itself.
- How much does it cost to fix P0301?
- If it's a spark plug, roughly $5–25 per plug DIY. An ignition coil typically runs $40–120 for the part. Injector cleaning or replacement is more, and mechanical causes (valves, head gasket) are a different conversation — but those are the minority of cases.
- Will P0301 clear itself?
- The light can turn off on its own if the misfire stops happening (for example, a one-time fuel quality issue), but the underlying cause usually doesn't heal. If the code returns after clearing it, something real is wrong.
- How do I know if it's the plug or the coil?
- Swap the cylinder 1 coil with another cylinder and clear the code. If the misfire moves with the coil (P0301 becomes P0303, for example), it's the coil. If it stays on cylinder 1, suspect the plug, injector, or compression.
- Where is cylinder 1?
- It depends on the engine. On most inline engines it's the cylinder nearest the front of the engine (the accessory belt end). On V engines it varies by manufacturer — check your repair manual or look up your engine's cylinder numbering diagram.