Code guide
P0303 — cylinder 3 misfire
This code isolates cylinder 3 — but the single most useful fact about it is whether a second code shows up on its paired cylinder at the same time. That one detail is the difference between two repairs and one.
Cylinder 3 · Misfire · P0301, P0302, P0304–P0312 are the same fault on other cylinders
The short version
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 it can destroy within minutes.
P0303 means the misfires were on cylinder 3 specifically. Same family as P0301 and P0302 — the last digit is the cylinder number, and an isolated misfire usually points at a component on that one cylinder: spark, fuel, or compression.
There's one question worth answering before chasing any of those, and it costs nothing: did P0302 set at the same time?
Reading codes, clearing them, and watching live data are all free in Fixiom — always, no credits and no subscription. The scan screen shows every stored code at once, so the pair check below is a five-second look.
P0302 and P0303 together
On engines with waste-spark (distributorless) ignition — common on four-cylinders built through the 2000s, and some V6s — one coil often fires two cylinders at once. On the typical inline-4 layout, cylinders 2 and 3 are frequently that pairing.
If your engine works this way and P0302 and P0303 are stored together, that's strong evidence for one shared coil rather than two unrelated cylinder problems. A failing coil-on-plug arrangement can't misfire two cylinders at once by coincidence — but a failing shared coil absolutely can. Buying two separate ignition coils, two sets of injectors, or diagnosing two independent faults is the classic wasted repair here; buying and testing the one shared coil first is the efficient move.
This is worth confirming, not assuming: verify your specific engine actually uses waste spark and that 2 and 3 are genuinely paired on it before treating the two codes as one story — the exact pairing is engine-specific.
P0303 alone
If cylinder 3 is the only misfire code — even on a waste-spark engine where it shares a coil with cylinder 2 — that argues against the shared coil, since a coil failing badly enough to set a code usually drags its partner along too, at least intermittently. Suspicion moves back to what belongs to cylinder 3 alone: its own plug, its own injector, a vacuum leak local to that intake runner, or compression on that cylinder specifically.
On engines with independent coil-on-plug ignition, this is simply the normal case — there is no partner cylinder to check, and the diagnosis is the same single-cylinder process as P0301.
The swap test, in brief
Move the suspect part to a different cylinder, clear the code, drive it the way it normally misfires, and re-scan. If the code follows the part, that's the fault. P0301's full walkthrough covers the procedure and how to read each outcome in detail — it applies here unchanged. One extra note for a shared-coil situation: swap the coil itself, not just a plug, since the coil is the component the pair rule points at.
Likely causes, in order
- Ignition coil — check the paired cylinder's code status first if your engine uses waste spark.
- Spark plug for cylinder 3 — worn, cracked, or fouled.
- Fuel injector for cylinder 3 — clogged, leaking, or electrically failing.
- A vacuum leak local to that cylinder's intake runner.
- Low compression on cylinder 3 — once spark and fuel are eliminated.
- Wiring or a connector at that cylinder's coil or injector.
Which one is cylinder 3?
Numbering follows the engine's own convention rather than physical front-to-back position, and it varies by manufacturer and layout. Confirm it for your specific engine in service information before you swap or remove anything — getting it wrong is a genuinely common way to waste an afternoon and draw the wrong conclusion.
What a phone can't see
What it genuinely gives you: the code naming the cylinder, whether a paired cylinder's code is stored alongside it, freeze frame conditions, per-cylinder misfire counts under Mode 06, live data while it misfires, and clear-and-re-scan for the swap test.
What it cannot do: a compression or leakdown test, inspecting a spark plug, bench-testing a coil to confirm it's actually failing, or telling you which physical wire goes where on your specific engine. Live data narrows it and points at the right part; hands on the car confirm it.
Common questions
What does P0303 mean?+
The engine computer detected misfires specifically on cylinder 3. An isolated single-cylinder misfire usually points at a component on that cylinder — spark, fuel, or compression — rather than something the whole engine shares.
Why do I have both P0302 and P0303?+
On engines with waste-spark (distributorless) ignition, one coil commonly fires two cylinders together, and on many inline-4 layouts that pairing is cylinders 2 and 3. Both codes together points strongly at that one shared coil rather than two separate cylinder faults — confirm your engine actually uses waste spark before assuming it.
If only P0303 is stored, does that rule out the shared coil?+
It leans that way. A shared coil failing badly enough to set a code usually drags its partner cylinder along too, at least sometimes. P0303 alone argues for something local to cylinder 3 specifically — its own plug, injector, a local vacuum leak, or compression on that cylinder.
Can I drive with a P0303?+
A steady light is generally not an emergency, though worth diagnosing promptly. A flashing light means an active misfire is putting raw fuel into the exhaust, which can destroy the catalytic converter within minutes — stop driving as soon as it's safe.
Related guides
- P0302 — Cylinder 2 misfire — and whether a shared ignition coil is the reason.
- P0301 — Cylinder 1 misfire — and the swap test that proves the cause in one drive.
- P0300 — Random misfire — and the per-cylinder counters that show which one it really is.
- Check engine light flashing — A flashing light is not the same warning as a steady one. What to do right now.
One code or two together. That single fact is half the diagnosis.
The scan screen shows every stored code at once — free in Fixiom, always. Check the pair before you buy the part.