What does the P0304 code mean?

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.

What are the most common causes of P0304?

The most common causes are: 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.

Is it safe to drive with a P0304 code?

Often for a short distance, but do not ignore it. P0304 can worsen and lead to further damage or poor running, so have it diagnosed soon.

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: Swap coil or plug to isolate, then replace it; Replace the cylinder 4 spark plug; Test or replace the cylinder 4 injector; Compression-test cylinder 4 if needed.

P0304: Cylinder 4 Misfire Detected

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

Compression tester - The end cylinder is where head gasket failures show first. This is how you catch it.
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.
Ignition coil - When the swap test moves the misfire with the coil.
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.