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DTC codes / P03xx · Ignition system and misfire / P0301

P0301Cylinder 1 Misfire Detected

P0301 means the engine computer has detected a misfire on cylinder 1, whose position the manufacturer defines, usually the cylinder at the timing-belt or timing-chain end of the engine. The computer watches crankshaft speed fluctuations and flags this code when cylinder 1 fails to contribute its share of power, usually within a specific RPM and load window.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaIgnition system and misfire

Symptoms you may notice

  • Often no symptoms besides the warning light, especially if the misfire is intermittent or only occurs under light load.
  • A noticeable shaking or rough idle that may smooth out as engine speed increases.
  • Loss of power and sluggish acceleration, particularly when climbing a hill or merging onto a highway.
  • A flashing check engine light during hard acceleration, which signals a misfire severe enough to damage the catalytic converter.
  • A raw fuel smell from the exhaust or black soot on the tailpipe when the misfire is constant.
  • Poor fuel economy and, on some vehicles, a slight shudder felt through the steering wheel or seat.

Common causes, most likely first

  1. A worn or fouled spark plug on cylinder 1, which is the most common cause on high-mileage engines.
  2. A failed ignition coil or plug boot dedicated to cylinder 1, especially on coil-on-plug systems.
  3. A clogged or leaking fuel injector for cylinder 1, delivering too little or too much fuel.
  4. A vacuum leak near cylinder 1, such as a cracked intake manifold gasket or a broken vacuum hose.
  5. Low compression on cylinder 1 from worn rings, a burned valve, or a failed head gasket.
  6. A faulty crankshaft position sensor or damaged reluctor wheel that corrupts the misfire detection signal.
  7. Mechanical valvetrain problems such as a collapsed lifter or a worn camshaft lobe on cylinder 1.

How to diagnose and fix it

  1. Scan all stored and pending codes and freeze-frame data to confirm the misfire is isolated to cylinder 1 and note the RPM and load conditions.
  2. Inspect the spark plug on cylinder 1 for fouling, wear, or a cracked insulator, and compare it to a known-good cylinder.
  3. Swap the ignition coil from cylinder 1 with a neighbouring cylinder and see if the misfire follows the coil, which confirms a bad coil without buying parts.
  4. Check fuel injector operation with a noid light or by swapping the injector to another cylinder and watching whether the misfire moves.
  5. Perform a compression test and, if compression is low, a cylinder leak-down test to rule out mechanical failure before replacing ignition parts.
  6. Smoke-test the intake system for vacuum leaks near cylinder 1 and repair any cracked gaskets or hoses.
  7. After repairs, clear the codes and drive the vehicle through the misfire monitor's enable criteria to confirm the code does not return.

What a P0301 misfire really tells you

  • A P0301 code does not always mean the spark plug or coil is bad; a leaking intake gasket or a sticking injector can set the same code, so swapping parts without testing wastes money.
  • On engines with coil-on-plug ignition, a single failed coil often sets P0301 and a companion code such as P0351, and replacing only the plug leaves the real fault in place.
  • A flashing check engine light with P0301 means raw fuel is reaching the catalytic converter and can melt it within minutes, so the vehicle should not be driven until the misfire is repaired.
  • Some vehicles set P0301 from a dirty mass airflow sensor or a failing fuel pump that leans out cylinder 1 first, so check fuel trims before condemning ignition components.
  • A misfire that only appears when the engine is cold often points to a worn valve guide or a weak coil, while a misfire that appears only when hot can indicate a failing injector or a cracked head.

Can I keep driving with P0301?

You can drive a short distance to a repair shop if the check engine light is steady and the engine runs smoothly, but avoid hard acceleration and heavy loads. If the light is flashing or the engine shakes violently, stop driving immediately because unburned fuel will destroy the catalytic converter.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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