Fixiom

Code guide

P0301 — cylinder 1 misfire

This code names the cylinder — which makes it one of the few faults where a genuinely conclusive test exists. Move the suspect part, drive it, and watch whether the misfire moves with it.

Read this first The short version The swap test Reading the result Likely causes in order Which one is cylinder 1? What a phone can't see

Cylinder 1 · Misfire · P0302–P0312 are the same fault on other cylinders

Read this first

If your check engine light is FLASHING, stop driving as soon as you safely can. A flashing light during a misfire means unburned fuel is reaching the catalytic converter, which can destroy it within minutes. That turns an inexpensive repair into an expensive one. A steady light is much less urgent.

The short version

P0301 means the computer detected misfires specifically on cylinder 1. The last digit of the code is the cylinder number, so P0302 is cylinder 2, P0303 is cylinder 3, and so on — everything on this page applies equally, just on a different cylinder.

Because the fault is isolated to one cylinder, the cause is almost always a component on that cylinder rather than something the whole engine shares. Three things live there: spark, fuel, and compression. Diagnosis is the process of working out which.

And because it's isolated, you can do something you can't do with a random misfire: move the suspect part to a different cylinder and see whether the problem moves with it. That's about as close to proof as diagnosis gets.

Reading codes, clearing them, and watching live data are all free in Fixiom — always. The swap test below needs a code clear and a re-scan, and neither costs credits.

The swap test

The logic is simple. If the ignition coil from cylinder 1 is faulty and you move it to cylinder 2, then the misfire should move to cylinder 2 with it — the code becomes P0302. If the misfire stays on cylinder 1, the coil was never the problem and you've eliminated it for the cost of a few minutes.

The same applies to injectors on engines where they're accessible.

The procedure, in principle:

  • Engine off and cool, ignition off. Coils sit on top of a hot engine and there's no reason to work on one that's just been running.
  • Swap the suspect component with the same component from a neighbouring cylinder. Note carefully which cylinder you moved it to.
  • Clear the code so you're starting from a blank slate. This is the step people skip, and without it the result is meaningless.
  • Drive it the way it normally misfires — from the freeze frame you already know the conditions that set the code. Cold-only misfires need a cold start, load-only misfires need load.
  • Re-scan and read which cylinder the code names now.
This page is about how to reason, not a repair procedure. How accessible the coils and injectors are varies enormously between engines — on some it's a single bolt and a connector, on others there's an intake manifold in the way. Check what's actually involved on your engine before committing, and if it's not straightforward, this is a very reasonable thing to hand to a shop with the diagnosis already in hand.

Reading the result

  • The misfire followed the part → you've found it. The component you moved is faulty. This is a genuine confirmation, not a guess.
  • The misfire stayed on cylinder 1 → the component you moved is fine. Eliminate it and move to the next suspect. This is real progress even though it feels like a dead end.
  • The misfire disappeared entirely → be careful here. Most often this means a connector was dirty or not fully seated and reseating it fixed the contact. Worth noting, and worth re-checking in a few hundred miles, because an intermittent connection can come back.
  • Now both cylinders misfire → you likely have two marginal components, or the cause is something shared and cylinder 1 was simply the weakest. Step back and treat it as a random misfire.

Likely causes, in order

  • Ignition coil for that cylinder. The most common single-cylinder misfire cause on modern coil-on-plug engines, and the easiest to swap-test.
  • Spark plug. A cracked insulator, a plug worn well past its gap, or one that's been fouled by something else going on. Plugs are consumable — if the set is overdue, the answer might be all of them rather than one.
  • Fuel injector for that cylinder. Clogged, leaking, or electrically failing. A stuck injector often produces a fuel smell or noticeably rough running at idle.
  • A vacuum leak local to that cylinder — an intake gasket leaking at one runner will lean out one cylinder specifically.
  • Low compression on that cylinder. The mechanical answer. Suspect it once spark and fuel have both been eliminated by swapping, and expect the misfire to be present under all conditions rather than only sometimes.
  • Wiring or connector trouble at that cylinder's coil or injector — chafed insulation, a corroded pin, a connector that's lost its grip.

Which one is cylinder 1?

Not necessarily the one nearest the front of the car. Cylinder numbering follows the engine's own convention, and it varies by layout and manufacturer: on a V6 or V8 the banks alternate in ways that aren't obvious by looking, and on transverse engines "front" is ambiguous to begin with.

Confirm the numbering for your specific engine before you pull anything. Swapping the wrong two coils and drawing a confident conclusion from it is a genuinely common way to waste an afternoon.

What a phone adapter can't see

What it genuinely gives you: the code naming the cylinder, freeze frame conditions from when it set, per-cylinder misfire counts under Mode 06, live data while it misfires, and the ability to clear and re-scan — which is what makes the swap test readable at all.

What it cannot do:

  • Compression or leakdown testing. The definitive mechanical tests, both requiring a gauge in the spark plug hole.
  • Inspect the plug. A plug from a misfiring cylinder tells a story — oil fouling, fuel fouling, damage — and reading it means removing it.
  • Bench-test a coil or an injector. Live data narrows; the bench confirms.
  • Do the swap for you. The test on this page needs hands, and how easy that is depends entirely on your engine.

Common questions

What does P0301 mean?+

The engine computer detected misfires specifically on cylinder 1. The last digit of the code is the cylinder number, so P0302 is cylinder 2 and so on. An isolated misfire usually points at a component on that cylinder — spark, fuel, or compression.

How do I know if it's the coil or the spark plug?+

Swap the suspect part with the same part from a neighbouring cylinder, clear the code, drive it the way it normally misfires, then re-scan. If the code moves to the other cylinder, the part you moved is faulty. If it stays put, that part is fine.

Can I drive with a P0301?+

If the check engine light is flashing, stop as soon as you safely can — raw fuel is reaching the catalytic converter and can destroy it in minutes. With a steady light it's less urgent, but a misfiring cylinder is worth sorting promptly.

Which cylinder is cylinder 1?+

Not necessarily the one closest to the front of the car. Numbering follows the engine's own convention and varies by layout and manufacturer, especially on V6 and V8 engines. Confirm it for your specific engine before removing anything.

Why did the misfire disappear after I reseated the coil?+

Usually a dirty or partially seated connector — reseating restored the contact. That's a real fix, but an intermittent connection can return, so it's worth re-checking after a few hundred miles rather than assuming it's solved.

Do I need to clear the code before the swap test?+

Yes, and it's the step people skip. Without clearing first you can't tell whether the code you're reading is new or left over from before the swap, which makes the whole result meaningless.

Move the part. See if the fault moves with it.

Reading codes, clearing them, and re-scanning are free in Fixiom — always. The swap test is one of the few things in diagnosis that gives you a straight answer.

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