P0301: misfire in cylinder 1

Code P0301 means the ECU has detected misfires — combustion events that didn't happen — in cylinder #1. A single missed spark is normal even on a new engine, but once misfires pile up, the check engine light comes on and, on many cars, fuel injection to that cylinder is cut to protect the catalytic converter.

P0300 is "random/multiple misfires" across cylinders; P0301–P030X point at a specific one. If P0301 comes with, say, P0302, the problem is shared (fuel, air) rather than one cylinder.

Symptoms

• Rough idle, shaking ("the engine runs on three");
• Lost power, especially under load;
• Higher fuel consumption;
• A flashing (not steady) check engine light — stop driving: unburnt fuel burns inside the catalytic converter and destroys it.

Common causes — from cheap to expensive

1. Spark plug — wear, fouling, wrong gap. The most common and cheapest cause; start here.
2. Ignition coil of cylinder 1 — insulation breakdown, cracks. Test by swapping with another cylinder: if the code "moves" (P0301 → P0303), it's the coil.
3. Plug wires (if fitted) — oxidation, tracking.
4. Injector — clogged or leaking: the cylinder runs rich or lean.
5. Vacuum leak — cracked hose, intake gasket: unmetered air reaches the cylinder.
6. Low compression — valves, rings. That one's expensive; in practice, though, 8 times out of 10 P0301 is plugs or a coil.
7. Bad fuel — misfires after a sketchy gas station often clear up once diluted with proper fuel.

What to do

1. Read the codes with an OBD2 scanner. Check the freeze frame — at what RPM and temperature it happened.
2. Inspect the cylinder 1 plug: color, deposits, gap. Compare with neighbors.
3. Swap coil 1 with another cylinder and drive a day: a "moving" code fingers the coil.
4. Still there — check the injector (bench/cleaning) and vacuum leaks (smoke test).
5. Clear the code only after fixing the cause: if it's still there, the light returns within a few drives.

You can read and clear P0301 without a shop

A Bluetooth ELM327 adapter costs about as much as two spark plugs and plugs into the OBD2 port (on most cars, under the steering column). PitStop has full diagnostics: code reading with plain-language explanations, clearing, live parameters and charts. It explains P0301 in human terms and walks you through the checks — like this article, but in your pocket.

← All articles