P0304 means the engine computer detected a misfire specifically in cylinder 4 — that cylinder is failing to produce its share of power, either because it isn’t getting a proper spark, isn’t getting the right amount of fuel, or can’t hold compression. Because the fault is isolated to one cylinder rather than scattered across the engine, you can identify the cause surprisingly cheaply by moving the coil and plug from cylinder 4 to another cylinder and seeing whether the misfire follows them.
What Does P0304 Mean?
The engine computer monitors tiny variations in crankshaft speed. Every successful combustion event gives the crankshaft a small acceleration; when one cylinder fails to fire, that acceleration is missing and the ECU can tell precisely which cylinder in the firing order didn’t contribute. P0304 is that report for cylinder 4.
Cylinder numbering is not universal, and cylinder 4 is not always the fourth one you’d count from the front of the engine — on transverse V6 layouts in particular, the numbering can jump between banks. Confirm your engine’s layout before you start pulling parts, or you’ll be testing a perfectly healthy cylinder.
A P0304 that stays on cylinder 4 is far simpler to chase than the random, roaming misfire our P0300 guide covers. When a misfire is confined to one cylinder, the cause is almost always a component belonging to that cylinder — its coil, its plug, its injector — and those are all things you can physically swap with a neighbour to test.
Common Symptoms
- A rough, shaking idle that smooths out somewhat as revs rise.
- Hesitation or a stumble under acceleration — see our car jerks when accelerating guide.
- Loss of power, since one cylinder of four is a quarter of the engine’s output.
- A flashing check engine light in more severe cases, which changes the urgency completely — our flashing check engine light guide explains why.
- Increased fuel consumption and a fuel smell from the exhaust, as unburnt fuel passes through.
A steady check engine light with P0304 means fix it soon. A flashing light means raw fuel is being dumped into a hot catalytic converter right now, which can destroy it in a short distance. Reduce load, stop driving as soon as it is safe to, and have the car recovered rather than driven.
Common Causes, Most Likely First
- A worn or fouled spark plug in cylinder 4 — the cheapest and most common cause by a wide margin.
- A failing ignition coil on that cylinder, on the coil-on-plug designs used by most modern engines.
- A clogged or failing fuel injector for cylinder 4, starving it of fuel.
- A damaged plug wire or boot on older distributor or coil-pack systems.
- A vacuum leak affecting that cylinder’s intake runner, which can lean out one cylinder specifically — worth checking if P0171 is also stored.
- Low compression in cylinder 4 — a burnt valve, worn rings or a head gasket leak. Least common, most expensive, and the reason a compression test belongs in the diagnosis rather than a parts cannon.
How to Diagnose It
- Swap the coil from cylinder 4 with the one from another cylinder, clear the code, and drive until it returns. If the code becomes P0303 or P0302, the coil is faulty and you’ve proved it for free.
- Do the same with the spark plug if the coil swap doesn’t move the misfire. Inspect the plug you removed while it’s out — heavy carbon fouling, oil, or a worn electrode gap all tell a story.
- Listen to the injector with a long screwdriver or stethoscope against it while the engine idles; a healthy injector makes a regular clicking tick. Swapping injectors between cylinders is the same trick as the coil swap if you suspect it.
- Check for stored fuel trim or lean codes, which point toward an intake leak on that runner rather than an ignition part.
- Run a compression test on cylinder 4 if plugs, coil and injector all check out. A cylinder reading well below its neighbours is a mechanical fault, and no amount of ignition parts will fix it.
Never work under a vehicle supported only by a jack. Use axle stands on firm, level ground, chock the wheels that stay down, and leave the car in gear or Park with the handbrake on. A jack lifts; stands hold.
Possible Fixes
| Cause | Fix | Typical cost |
|---|---|---|
| Worn or fouled spark plug | Replace plugs (ideally the full set) | $20 – $150 |
| Failed ignition coil | Replace the coil | $60 – $300 |
| Clogged fuel injector | Clean or replace the injector | $100 – $450 |
| Damaged plug wire/boot | Replace the wire set | $40 – $150 |
| Vacuum leak at the intake runner | Replace the gasket or hose | $50 – $400 |
| Low compression (valve, rings, head gasket) | Engine repair | $800 – $3,000+ |
What usually doesn’t work: replacing all four coils and all four plugs at once before testing anything. It sometimes fixes the car, but it also hides which part was actually failing, and it’s poor value when a five-minute coil swap would have told you for nothing. That said, if your plugs are near the end of their service interval anyway, replacing the full set while you’re in there is reasonable.
Can I Drive With P0304?
Short trips only, and not at all if the check engine light is flashing. A steady light with a mild misfire is tolerable for a short journey to a workshop, though you should expect reduced power and worse fuel economy. The real risk is the catalytic converter: unburnt fuel from a misfiring cylinder passes into a converter operating at several hundred degrees, and sustained misfiring can overheat and destroy it — turning a $60 coil into a repair many times that. If the light flashes, stop driving as soon as it is safe. See our flashing check engine light guide.
When to See a Mechanic
Promptly if the light is steady, immediately if it’s flashing. If you’ve already swapped the coil and plug and the misfire stayed on cylinder 4, a compression test is the next step and it’s worth having done properly. Compare with our P0301 guide for the cylinder 1 equivalent and our P0300 guide if misfires start appearing across several cylinders instead. More in our engine problems section.
Frequently Asked Questions
What does the P0304 code mean?
The engine computer has detected that cylinder 4 is misfiring — failing to produce its share of power because of a spark, fuel or compression problem in that specific cylinder.
Which cylinder is cylinder 4?
It depends on your engine’s layout, and it isn’t always the fourth cylinder counting from the front. Transverse V6 engines in particular number cylinders across both banks. Check your engine’s firing order and numbering before removing parts.
Can I fix a P0304 myself?
Often yes. Swapping the ignition coil from cylinder 4 to another cylinder and seeing whether the code follows it costs nothing and identifies a failed coil definitively. The same test works for spark plugs and injectors.
Is it safe to drive with a P0304 code?
Short trips only with a steady check engine light, and not at all if the light is flashing. Sustained misfiring pushes unburnt fuel into a hot catalytic converter and can destroy it, turning a cheap repair into an expensive one.
How much does it cost to fix P0304?
Frequently $20 to $150 for spark plugs or $60 to $300 for an ignition coil. Only if compression turns out to be low does it become a major expense.
