P0304: Cylinder 4 Misfire Detected

P0304 quick answer

P0304 means a misfire was detected in cylinder 4. Like the other single-cylinder misfire codes, it points to the coil, plug, or injector for that cylinder, or to low compression inside it.

Severity: Moderate
Can I drive? Short-term, usually yes, if the check-engine light is steady - a single-cylinder misfire rarely leaves you stranded.
Typical fix: $100 - $450

Also written as PO304 or P304.

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What P0304 means

P0304 means a misfire was detected in cylinder 4. Like the other single-cylinder misfire codes, it points to the coil, plug, or injector for that cylinder, or to low compression inside it. Swapping the suspected coil or plug to another cylinder and rescanning is the fastest way to confirm the culprit before buying parts.

Symptoms of P0304

Rough idle
Hesitation and stumble
Flashing check-engine light during a hard misfire
Power and fuel-economy loss

Is it safe to drive with P0304?

Short-term, usually yes, if the check-engine light is steady - a single-cylinder misfire rarely leaves you stranded. Do not put off diagnosis, though: driving on it for long washes oil off the cylinder wall and lets unburned fuel reach the catalytic converter. If the light starts flashing, stop driving - that means the misfire has become severe enough to damage the converter.

Common causes of P0304

Bad ignition coil on cylinder 4
Worn or fouled cylinder 4 spark plug
Clogged or leaking cylinder 4 fuel injector
Vacuum leak near cylinder 4
Low compression in cylinder 4

Listed roughly most-likely first. Confirm with a scan tool and the freeze-frame data before replacing parts.

What the P0304 fix costs

Fix	DIY parts	At a shop
Replace the cylinder 4 ignition coil	$25 - $90	$100 - $250
Replace the cylinder 4 spark plug	$8 - $25	$60 - $150
Test or replace the cylinder 4 injector	$40 - $150	$150 - $450

Ranges are rounded and for the US market only - actual price depends on region, shop labor rates, and the exact make and model.

What a new ignition coil costs, if testing points to it

Parts that commonly fix P0304

Ignition coil - Cause #1 for a single-cylinder misfire - prove it with the coil swap test first
Spark plugs - Cause #2: worn or fouled plugs. Replace as a set, in the heat range your engine specifies
Inline spark tester - Shows whether the coil is delivering spark to that cylinder in seconds
Dielectric grease - A thin coat inside the coil boot stops the arcing and moisture that cause repeat misfires
Compression tester - For when the misfire stays put after coil and plug swaps - rules low compression in or out
OBD-II scanner - Reads and clears codes, and shows live data so you fix the cause rather than the symptom

These open an Amazon search - add your year, make and model, because most of these parts are fitment-specific.

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What P0304 actually means

P0304 reports a misfire on cylinder 4. On a four-cylinder engine this is the cylinder at the far end from the accessory belt, usually against the firewall or the transmission bellhousing, and that position matters - it tends to run slightly hotter and it is where coolant flow is weakest on some designs. The diagnosis follows the same swap-and-confirm method as any cylinder-specific misfire, but cylinder 4 has one distinctive pattern worth knowing: on engines prone to head gasket failure, the end cylinder is frequently the first to show it.

How to diagnose P0304 step by step

Step 1: Swap the cylinder 4 coil with cylinder 3 and clear Drive until the code returns. Migration to P0303 confirms the coil. No migration means the coil is good. Start here every time - it is free and it is definitive.
Step 2: Swap the injector next if the coil proved good Same procedure with the cylinder 4 injector. This isolates a clogged or failing injector without committing to a full set, which on direct-injection engines is a substantial saving.
Step 3: Examine the cylinder 4 plug for coolant signature Beyond the usual oil and fuel checks, look specifically for a plug that appears steam-cleaned - unusually white and clean compared to its neighbours. That is the signature of coolant entering the cylinder, and on end cylinders it points at a head gasket.
Step 4: Compression test cylinder 4 against cylinder 3 The adjacent cylinder is your control. A significant deficit on the end cylinder specifically, combined with any coolant loss you cannot account for, is the classic head gasket pattern rather than a simple worn part.
Step 5: Check the coolant for combustion gases if compression is low A block tester kit changes color in the presence of exhaust gas in the coolant. This is a ten minute test that definitively separates a head gasket from worn rings or a burnt valve, and it decides the entire scope of the repair.
Step 6: Inspect the harness routing near the firewall The cylinder 4 coil connector sits in the hottest, tightest, least accessible corner of the bay on many engines. Heat-hardened connectors and chafed wiring there cause intermittent misfires that survive every part you replace.

The most common P0304 misdiagnosis

Treating a cylinder 4 misfire as automatically an ignition problem misses the pattern that matters. On the end cylinder, an unexplained coolant loss alongside this code is a head gasket signature, and continuing to drive it turns a gasket job into a warped head.

Tools you need for P0304

Combustion leak block tester - Definitively confirms or rules out a head gasket in about ten minutes.
Spark plug socket with rubber insert - Removing the rear plug without cracking it in a cramped bay.
Flexible ratcheting wrench set - Cylinder 4 against the firewall rarely allows a straight tool approach.

Parts you may need for P0304

Spark plugs, full set - Always a set. A single new plug among worn ones just moves the imbalance.
Head gasket set - Only after a block test confirms combustion gases in the coolant.

P0304: DIY or pay a shop?

Difficulty: Easy to Moderate. Time: 0.5 - 3 hours. DIY parts: $40 - $200. Shop total: $150 - $450.

Coil and plug work on cylinder 4 is straightforward if you can reach it, though the firewall side of a transverse engine tests your patience and your wrist. The real branch point is the compression result: normal compression means a cheap DIY afternoon, while a confirmed head gasket is a major job that belongs at a shop for almost everyone.

How to stop P0304 coming back

Keep the cooling system serviced and never ignore coolant loss. End cylinders suffer first when cooling is marginal, and the overheating events that kill head gaskets are usually preceded by months of slow coolant loss nobody investigated. Old coolant also corrodes the gasket sealing surfaces from the inside. Log coolant top-offs in ServiceSage. A slow leak is invisible one cup at a time and obvious in a maintenance history - which is what lets you fix it before a gasket goes. Track this maintenance free.

P0304 FAQ

How much does it cost to fix P0304?

Typical US independent-shop repair cost for P0304 runs about $100 - $450, depending on the root cause. Common fixes: Replace the cylinder 4 ignition coil (about $100 - $250 at a shop); Replace the cylinder 4 spark plug (about $60 - $150 at a shop); Test or replace the cylinder 4 injector (about $150 - $450 at a shop).

Will P0304 clear itself?

No, not on its own. P0304 will keep coming back every time the relevant monitor re-runs until the underlying cause is actually repaired - clearing it with a scan tool without fixing what triggered it only delays the light coming back on.