P0301

00:00What this code means

Code P0301. Cylinder 1 Misfire Detected.

In plain English: the computer measured a weak or missing power stroke from cylinder one, but it has not told you whether the cause is spark, fuel or compression.

If the check engine light is flashing, stop driving and arrange a tow. A flashing light means an active misfire is sending unburned fuel into the catalytic converter and can overheat it quickly. With a steady light and a smooth-running engine, drive only as far as needed for diagnosis.

00:34How the test actually works

Here's what your engine computer is actually measuring.

The computer does not see the flame inside cylinder one. It watches crankshaft speed. Every successful power stroke accelerates the crank slightly; when cylinder one should fire and the crank slows instead, the crankshaft position signal lets the computer attribute that lost contribution to cylinder one.

The computer keeps a misfire count for each cylinder and stores P0301 when cylinder one's count crosses its calibrated limit. That identifies the cylinder, not the failed part. Cylinder one still needs spark, the correct fuel charge and enough compression, and a failure in any one of those produces the same code.

So the code is not a part failure report. It's a performance measurement that came back under spec.

01:26Symptoms

What you'll usually notice, beyond the light itself.

Shaking or uneven idle. One missing power stroke is easiest to feel when the engine is turning slowly.

Flashing check engine light. The urgent sign. Stop driving before raw fuel overheats the converter.

Hesitation under acceleration. A weak coil or plug often fails only when cylinder pressure rises under load.

Raw-fuel smell at the exhaust. Fuel entered cylinder one but did not burn.

Reduced power and fuel economy. The remaining cylinders are carrying the load while one wastes its charge.

02:11Most likely causes, ranked

Now the part you came for: what actually causes this, ranked by how often it turns out to be the culprit.

28 percent of the time, it's this: Worn or fouled spark plug on cylinder one. Especially likely when the plugs are overdue or the insulator is oil- or fuel-fouled.

27 percent of the time, it's this: Weak ignition coil or damaged plug lead. The swap test identifies it for free: if the code moves with the coil, the coil is the cause.

17 percent of the time, it's this: Restricted or failed fuel injector. The misfire stays on cylinder one after ignition parts move, and an injector balance test singles it out.

13 percent of the time, it's this: Low compression from a valve, ring or gasket fault. The expensive branch, tested after spark and fuel because it is less common and takes more time.

10 percent of the time, it's this: Local intake or vacuum leak near cylinder one. A runner gasket leak can lean out the nearest cylinder without affecting the rest.

5 percent of the time, it's this: Connector, wiring or engine-computer driver fault. Look for a missing coil or injector command before blaming the computer itself.

Notice that the most expensive repair is not the most likely one. That matters, because the cheap checks come first.

03:38The measurement that decides it

This is the single measurement that tells you whether you need the expensive part or the cheap one.

First confirm cylinder one is the counter climbing. Then move its ignition coil to a known-good cylinder, clear the counters, and repeat the same idle or loaded test. Mark every part so you know exactly what moved.

Before the swap, cylinder one climbs while the comparison cylinder stays flat. That confirms a repeatable cylinder-one fault rather than a random event.

If the climbing count moves to the cylinder that received coil one, the fault moved with the part. The coil has failed, and you can replace it without guessing.

If the count stays on cylinder one after the coil moves, the coil is innocent. Repeat with the spark plug, then test injector command and balance, and save compression for last.

04:32Diagnostic order

So here is the order to actually test this, cheapest and most likely first.

Step 1. Is the check engine light flashing or is the engine shaking badly? A flashing light is a stop signal, not permission to keep testing on the road. Shut it down before unburned fuel overheats the converter, then diagnose it where the engine can be run safely.

Step 2. When coil one is swapped with another cylinder, does the misfire move? Confirm the cylinder numbering first, mark both coils, clear the counters and repeat the same test. If P0301 becomes P0302 after coil one moved to cylinder two, the coil convicted itself.

Step 3. Does the fault move when the cylinder-one spark plug is inspected and swapped? Look for a cracked insulator, excessive gap, oil or fuel fouling. Move the plug to a different known-good cylinder and test again. Replace the full set if they are all at the service interval.

Step 4. Does cylinder one's injector pass command and balance tests? Confirm the connector has power and a pulsed command, then compare the fuel-pressure drop from each injector using the factory procedure. No pulse points to wiring; a weak pressure drop points to a restricted injector.

Step 5. Is cylinder-one compression close to its neighbours? Test it last. A cylinder well below the others needs a leak-down test to separate an intake valve, exhaust valve, rings or head gasket. Use the engine maker's specification rather than one universal number.

Move one known part at a time and see whether the fault follows it. That is how a five-minute test replaces a pile of guessed parts.

06:27Repair costs

What this costs to put right, in US dollars, parts and labor.

Spark plug: 5 to 30 dollars in parts, plus about 0.5 hours of labor. The cheapest likely outcome; replace the set when all are due.

Ignition coil: 35 to 250 dollars in parts, plus about 0.5 hours of labor. The swap test tells you whether this money needs to be spent.

Fuel injector, single: 60 to 350 dollars in parts, plus about 1.5 hours of labor. Test command and balance before replacing it.

Intake gasket or vacuum hose: 8 to 140 dollars in parts, plus about 1.5 hours of labor. A local leak is uncommon but can target the nearest runner.

Valve, ring or head-gasket repair: 400 to 3000 dollars in parts, plus about 8.0 hours of labor. The expensive branch and the reason compression comes last.

Spark plugs and coils account for more than half of these cases, and both can be proven with a swap before you spend anything. Fuel and compression only move to the front after the misfire refuses to follow the ignition parts.

07:43Recap and related codes

Quick recap. P0301 means the computer localised a misfire to cylinder one; it did not name a bad coil. Stop if the light flashes. Otherwise swap the coil, then the plug, and watch whether the misfire moves. If it stays on cylinder one, test the injector and compression in that order.

If your scan tool showed a related code alongside this one, check those too: P0300, P0302, P0201, P0171.

Codes are on screen and linked below.