P0304 Trouble Code: Meaning, Causes, Symptoms & Fixes

One-line meaning: P0304: Cylinder 4 Misfire Detected.

What the P0304 Code Means

When your scanner shows the p0304 engine code, it means your car’s engine computer has detected a misfire in cylinder number 4 specifically. The full definition is “Cylinder 4 Misfire Detected” (p0304 cylinder 4 misfire detected). A misfire happens when the air-fuel mixture inside that cylinder fails to ignite properly, so the cylinder misses its power stroke and the crankshaft spins unevenly. Unlike the broader P0300 random misfire code, the p0304 code tells you exactly which cylinder is at fault, which makes diagnosis much more focused.

Here is how the computer knows. The crankshaft position sensor watches the crankshaft’s rotation speed with extreme precision. Each cylinder’s combustion gives the crankshaft a tiny push. When cylinder 4 fails to fire properly, even for a fraction of a second, the crankshaft hesitates at that point in its rotation. The computer counts these hesitations, and when they happen often enough on cylinder 4, it stores the p0304 code and turns on the check engine light.

P0304 (p0304 obd ii) is a generic powertrain code, which means it has the same meaning on every make and model. Its siblings P0301, P0302, P0303 and the rest of the P0301-P0312 range point at other individual cylinders, while P0300 means misfires are scattered randomly. Because the p0304 code names one cylinder, the usual suspects narrow to that cylinder’s spark plug, ignition coil, fuel injector, wiring, or compression.

Symptoms of the P0304 Code

The symptoms of a check engine light code p0304 range from a subtle shake to a car that barely runs. Here is what drivers most often notice:

• Rough idle. The engine shakes or vibrates at stoplights because cylinder 4 is not pulling its weight.
• Hesitation or stumbling under acceleration. The car feels like it momentarily loses power when you press the gas, then catches up.
• Flashing check engine light. A steady light means the misfire is occurring; a flashing light means a severe misfire is happening right now and raw fuel may be reaching the catalytic converter.
• Reduced fuel economy. An underperforming cylinder wastes fuel, so you will find yourself filling up more often than usual.
• Lack of power. The car feels sluggish, particularly going uphill or merging onto highways.
• Engine vibration. A steady shake you can feel through the steering wheel or seat, especially at idle.
• Hard starting. The engine cranks longer than normal before catching, especially on cold mornings.
• Unburned fuel or rotten-egg smell from the exhaust. A misfiring cylinder sends unburned fuel into the exhaust, which can overheat the catalytic converter and produce a sulfur-like smell.

If your check engine code p0304 came with a flashing light, treat it as urgent. A flashing light is the computer’s way of warning you that the catalytic converter is at risk of damage.

Common Causes of P0304

The p0304 code has a short list of usual suspects, all tied to cylinder 4. In rough order of how often they are the culprit:

• Worn spark plug (cylinder 4). The number one cause. A plug with rounded electrodes, excessive gap, or heavy carbon deposits cannot make a strong enough spark to ignite the mixture.
• Failing ignition coil or coil boot (cylinder 4). A weak coil produces a weak spark. Cracked coil boots let spark energy leak to ground instead of reaching the plug.
• Clogged or failing fuel injector (cylinder 4). An injector that dribbles instead of spraying a fine mist starves its cylinder of properly atomized fuel, so the mixture stays too lean to fire.
• Vacuum leak near cylinder 4. A leaking intake manifold gasket or cracked hose near the cylinder 4 runner lets unmetered air in and leans out that cylinder’s mixture.
• Wiring or connector problems. Damaged wiring, corroded pins, or a loose connector at the cylinder 4 coil or injector interrupts the signal intermittently.
• Low compression in cylinder 4. Worn piston rings, a leaking valve, or a blown head gasket reduce compression. Less common but serious, since it affects the mechanical ability of the cylinder to fire.
• Low fuel pressure. A weak fuel pump or clogged fuel filter can starve one cylinder first, especially under load.
• Incorrect cam or crank timing. A stretched timing chain or jumped timing belt can throw one cylinder’s timing off. Rare for a single-cylinder code, but possible.

How to Diagnose P0304

Diagnosing the p0304 engine code is easier than a random misfire because you know exactly where to look. Follow this order:

1. Read all codes and freeze-frame data. Check whether companion codes are present. A p0304 code paired with P0300 means cylinder 4 is the worst offender in a wider misfire problem. Note the freeze-frame RPM, load, and coolant temperature from when the code set.
2. Swap the ignition coil. Move the cylinder 4 coil to another cylinder, clear the code, and drive. If the misfire follows the coil (the code changes to that cylinder), the coil is bad. This free test saves you from buying parts blindly.
3. Inspect the spark plug. Pull the cylinder 4 plug and read it. Rounded electrodes, oil fouling, or heavy deposits mean replace the set. Also check the gap against the spec in your owner’s manual.
4. Test the fuel injector. Listen to the cylinder 4 injector with a mechanic’s stethoscope for a steady click, or swap it with another cylinder’s injector the same way you swapped the coil. You can also check injector resistance with a multimeter against spec.
5. Check wiring and connectors. Inspect the coil and injector connectors for corrosion, loose pins, and damaged insulation. Wiggle-test the harness while the engine runs to catch intermittent faults.
6. Compression test if the basics check out. If the plug, coil, injector, and wiring all test good, run a compression or leak-down test on cylinder 4 and compare it with the other cylinders. Low compression here means internal engine work.

How to Fix the P0304 Code

The right fix for the engine code p0304 depends entirely on what your diagnosis found. Here are the fixes matched to causes:

• Replace the spark plug. If the cylinder 4 plug is worn or the set is overdue, replace the full set (not just one). Use the exact plug type your owner’s manual specifies.
• Replace the faulty ignition coil or boot. If the coil-swap test pointed at the coil, replace it. Replace any cracked boot at the same time; on coil-on-plug systems the boot is often the real failure point.
• Clean or replace the fuel injector. Try a quality fuel system cleaner first for a mildly dirty injector. A truly failed injector needs replacement.
• Repair wiring and connectors. Clean corroded terminals, repair damaged insulation, and make sure connectors seat fully and lock.
• Fix vacuum leaks. Replace a leaking intake manifold gasket or cracked hose near cylinder 4. A smoke test is the fastest way to find small leaks, and small hose fixes cost almost nothing.
• Address compression issues. Valve jobs, ring replacement, or head gasket repair are major mechanical work for a professional shop.

After any repair, clear the code with your scanner and drive normally. The p0304 code should stay off through several drive cycles if the fix worked.

P0304 Repair Costs

How much it costs to fix a p0304 engine code depends on which cause turns out to be yours. Here are realistic ranges:

Repair	Cost Range
Spark plug replacement (full set)	$80 – $250
Ignition coil replacement (cylinder 4)	$70 – $250
Coil boot replacement	$20 – $60
Fuel injector cleaning	$50 – $150
Fuel injector replacement (cylinder 4)	$150 – $400
Wiring or connector repair	$100 – $300
Intake manifold gasket	$200 – $600
Compression-related engine repair	$1,000 – $3,500+

The typical total for a check engine light code p0304 falls between $100 and $600. A plugs-and-coil fix sits at the low end; injector or internal engine work climbs from there.

DIY vs Mechanic Verdict

DIY it if: the cause is the spark plug, the ignition coil, or a coil boot on cylinder 4. These are all doable at home with basic hand tools, and most p0304 code cases are solved at this level.

See a mechanic if: you need injector flow testing, a compression or leak-down test, wiring diagnosis deep in the harness, or internal engine work.

Verdict: Start DIY with the coil-swap test and a plug inspection. If that does not solve it, pay for a professional diagnosis before buying more parts.

Can You Drive With a P0304 Code?

It depends on whether the check engine light is steady or flashing. With a steady light and mild symptoms, you can drive gently to a shop, but get it diagnosed soon because a misfiring cylinder dumps raw fuel into the exhaust and can overheat the catalytic converter. With a flashing light, stop driving. A flashing light means a severe misfire is actively dumping raw fuel into the catalytic converter, which can melt it and turn a cheap fix into a very expensive one.

P0304 on Popular Vehicles

• Honda (Accord, Civic, CR-V): The p0304 honda code (honda p0304, p0304 code honda) most often traces to a failing coil-on-plug on cylinder 4. On higher-mileage Hondas, tight exhaust valves can also cause a single-cylinder misfire, so have the valve clearance checked.
• Toyota (Camry, Corolla, RAV4): For p0304 toyota (toyota p0304, p0304 code toyota), start with the coil and plug, then the injector. On direct-injection models, carbon buildup on the intake valves can contribute to single-cylinder misfires.
• Nissan (Altima, Sentra, Rogue): The p0304 nissan code (nissan p0304, p0304 code nissan) frequently involves ignition coils, a known weak point on several Nissan engines. Swap the coil first before buying anything.
• Ford (F-150, Escape, Fusion): A p0304 ford (ford p0304, p0304 code ford) is classic coil-on-plug failure territory. Moisture in the plug well from a leaking cowl seal or a car wash can kill a coil on cylinder 4, so check the plug tube for water.
• Chevy (Cruze, Equinox, Silverado): The p0304 chevy code (chevy p0304, p0304 code chevy) often comes down to plugs and coils. On the Cruze, a failing valve cover with the integrated PCV valve can also cause misfires, so do not overlook crankcase ventilation.
• Dodge / Jeep: A p0304 dodge or Jeep with high miles deserves a compression check sooner rather than later if plugs and coils test good, since these engines can develop valve seat issues.

P0304 vs P0300: What Is the Difference?

Both are misfire codes, but they point in different directions. P0304 means cylinder 4 specifically is misfiring, which points at that cylinder’s plug, coil, injector, wiring, or compression. P0300 means misfires are happening randomly across multiple cylinders, which suggests a shared-system cause like low fuel pressure, a large vacuum leak, or a failing mass airflow sensor. If you have a p0304 code plus a P0300, focus on cylinder 4 first; fixing it often clears the P0300 as well. For more on misfire diagnosis, see our misfire trouble codes guide.

Related Codes

• P0300: Random/Multiple Cylinder Misfire Detected (often appears alongside P0304)
• P0301 / P0302 / P0303: Cylinder-specific misfire codes for cylinders 1, 2, and 3
• P0171 / P0174: System Too Lean codes that can cause misfires

Video Walkthrough

Watch O’Reilly Auto Parts explain what cylinder misfire codes P0301 through P0312 mean, including the P0304 code, in the video above.

Frequently Asked Questions

Conclusion

The p0304 code is one of the friendlier check engine lights you will meet, because it tells you exactly which cylinder needs attention. Most of the time, a p0304 engine code is a quick, affordable repair: a new spark plug or ignition coil on cylinder 4. Work through the diagnosis steps in order, fix what you find, and clear the code to confirm the repair worked.

About the author: Written by the OBD2 Code Hub editorial team. This guide is for informational purposes. If you are unsure about any repair, consult a qualified mechanic.