Code guide
P0304 — cylinder 4 misfire
This code isolates cylinder 4 — but on a V6 or V8, which physical cylinder carries that number is a service-information lookup rather than a guess, and getting it wrong before opening anything up is an easy way to waste an afternoon.
Cylinder 4 · Misfire · P0301–P0303, P0305–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.
P0304 means the misfires were on cylinder 4 specifically. Same family as P0301 through P0312 — the last digit is the cylinder number, and because the fault is isolated to one cylinder, the cause is almost always something local: spark, fuel, or compression on that cylinder.
The one thing worth doing before touching anything is the same for every cylinder-specific code, but it matters most here: confirm which physical cylinder your engine calls "4."
Reading codes, clearing them, and watching live data are all free in Fixiom — always, no credits and no subscription.
Why cylinder 4 confuses people
On an inline four-cylinder, cylinder 4 is simply the last one in the row — straightforward, if you still confirm which end is "1."
On a V6 or V8, it's a different situation. Cylinder-numbering conventions are set by the manufacturer, and they genuinely don't agree with each other: some number straight down one bank before starting the other (1-2-3-4 on the left, 5-6-7-8 on the right), others alternate bank by bank (odd numbers on one side, even on the other), and cylinder 4 lands on a different bank depending on which convention your engine uses. There isn't a universal rule that works across brands, which is exactly why guessing from a forum post about a different manufacturer's engine is a common way to pull the wrong coil.
Confirm it in service information for your specific engine before you swap or remove anything. It takes a minute and it's the difference between testing the right part and testing the wrong one on the wrong bank.
Waste-spark pairing on four-cylinders
On inline-4 engines with waste-spark (distributorless) ignition — common through the 2000s — one coil often fires two cylinders together, and the usual companion pairing on that layout is cylinder 4 with cylinder 1, alongside 2 paired with 3 (the pairing P0303 covers in depth). If your engine works this way, check whether P0301 is stored at the same time — both together points at one shared coil rather than two separate cylinder faults. P0304 alone argues against that and points back at something specific to cylinder 4.
On engines with coil-on-plug ignition — the majority of engines built more recently — each cylinder has its own independent coil, and this pairing question doesn't apply.
The swap test, in brief
Once you're certain which physical cylinder is 4, the test is the same as any single-cylinder misfire: move the suspect coil or plug 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 result, unchanged here.
Likely causes, in order
- Ignition coil for cylinder 4 — on a waste-spark engine, check cylinder 1's code status first.
- Spark plug for that cylinder — worn, cracked, or fouled.
- Fuel injector for cylinder 4 — clogged, leaking, or electrically failing.
- A vacuum leak local to that cylinder's intake runner — on a V-engine, check the correct bank once you've confirmed which one carries cylinder 4.
- Low compression on cylinder 4 — once spark and fuel are eliminated.
- Wiring or a connector at that cylinder's coil or injector.
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: tell you which physical bank and position your manufacturer calls cylinder 4 — that's a service-information lookup, not a live-data reading. It also can't run a compression or leakdown test, inspect a spark plug, or bench-test a coil. Live data narrows it to a short list; confirming the numbering and the part is hands on the car.
Common questions
What does P0304 mean?+
The engine computer detected misfires specifically on cylinder 4. An isolated single-cylinder misfire usually points at a component on that cylinder — spark, fuel, or compression — rather than something the whole engine shares.
Which cylinder is cylinder 4?+
On an inline-4, the last cylinder in the row. On a V6 or V8, it depends entirely on the manufacturer's numbering convention — some number straight down one bank before the other, some alternate bank by bank — and cylinder 4 lands on a different physical bank depending on which. Confirm it in service information for your specific engine before removing anything.
Why do I have both P0301 and P0304?+
On four-cylinder engines with waste-spark (distributorless) ignition, one coil often fires two cylinders together, and the usual pairing on that layout is cylinder 1 with cylinder 4. Both codes together points at that one shared coil rather than two unrelated cylinder faults — worth confirming your engine actually uses waste spark first.
Can I drive with a P0304?+
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
- P0301 — Cylinder 1 misfire — and the swap test that proves the cause in one drive.
- P0303 — Cylinder 3 misfire — and how a second code on its partner cylinder changes the diagnosis.
- 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.
Confirm the cylinder. Then run the test.
Codes, freeze frames, live data, and Mode 06 misfire counts are free in Fixiom — always. Get the numbering right before anything comes apart.