P0304 Code — Cylinder 4 Misfire Detected (Causes & How to Fix)

Quick Check – 60‑Second Diagnosis

Your check‑engine light is flashing, and the OBD‑II scanner shows P0304 – Cylinder 4 Misfire Detected. Before you open the hood, do a rapid visual scan:

1. Gas cap – make sure it clicks tight. A loose cap can throw off fuel pressure and trigger a misfire code.
2. Battery terminals – clean any corrosion; a weak battery can cause low coil voltage.
3. Spark plug wire/coil connector – look for cracked boots or loose pins.
4. Engine oil level – low oil can cause excessive wear on piston rings, leading to compression loss.

If anything looks out of place, fix it, clear the code with your scanner, and take the car for a short test drive. If the code returns, move on to the detailed steps below.

Safety note: Always disconnect the negative battery terminal before removing ignition components. The engine and exhaust can stay hot for several minutes after shutdown.

Symptoms You’ll Notice While Driving

A cylinder‑4 misfire creates a distinct set of driver‑visible cues:

If you experience any of these, especially a flashing light, stop driving and address the issue promptly to avoid catalytic converter damage.

Root Causes – What Triggers This Code

While each vehicle is unique, the underlying mechanisms that cause a cylinder‑4 misfire fall into three broad categories:

1. Ignition‑System Failures

• Faulty spark plug – worn electrodes, fouling, or an incorrect gap.
• Bad ignition coil – loss of primary or secondary voltage.
• Damaged spark plug wire or coil‑on‑plug boot – cracks, moisture ingress, or corrosion.

2. Fuel‑Delivery Problems

• Clogged or leaking fuel injector – insufficient fuel spray or a short circuit.
• Low fuel pressure – caused by a weak pump or a clogged fuel filter.
• Incorrect fuel trim – the ECU may be enriching or leaning out too much for cylinder 4.

3. Mechanical Issues

• Low compression – worn piston rings, a cracked head gasket, or a bent valve.
• Timing chain/belt stretch – misaligned cam timing can prevent proper valve opening.
• Vacuum leak near the intake runner for cylinder 4 – unmetered air throws off the air‑fuel ratio.

Understanding which group the problem belongs to will guide you through the diagnostic steps efficiently.

Step‑by‑Step Diagnosis & Repair

The following procedure moves from the simplest, least invasive checks to the more involved mechanical tests. Perform each step, clear the code, and retest before moving to the next.

1. Quick Visual Scan (already described above)

• Good result: All connections tight, gas cap sealed, no obvious damage.
• Bad result: Loose coil boot or cracked plug wire – re‑install or replace the damaged part, clear the code, and test drive.

2. Read Freeze‑Frame Data

1. Connect your OBD‑II scanner and select “Freeze Frame” for the P0304 entry.
2. Record Engine RPM, Load, Fuel Trim, and Ignition Timing at the moment of the misfire.

• Good result: RPM around 800‑900, fuel trim within ±5 %, timing near spec (usually 10‑15° BTDC).
• Bad result: Extreme fuel trim (±15 % or more) or timing far off spec suggests a fueling or timing issue that needs deeper inspection.

3. Spark Plug Inspection

1. Remove the spark plug from cylinder 4 using a 5/8‑inch spark plug socket.
2. Examine the tip:

• Fouled (black, oily) – may indicate a rich mixture or oil burning.
• Cracked porcelain – replace immediately.
• Normal appearance – proceed to resistance test.

3. Measure resistance with a digital multimeter set to the ohms (Ω) range.

• Good: A few kilo‑ohms (typical range for copper plugs).
• Bad: Open circuit (infinite resistance) or very low resistance (shorted).

Repair: Replace the plug if fouled, cracked, or out of spec. Re‑gap to the manufacturer’s specification (usually 0.6‑0.8 mm). Re‑install with a torque wrench set to the recommended torque (often 13‑15 lb‑ft for most passenger cars).

4. Ignition Coil Swap Test

1. Locate the coil for cylinder 4 (often a coil‑on‑plug).
2. Disconnect the electrical connector and remove the coil.
3. Install the coil from cylinder 2 (or any other cylinder) into the cylinder‑4 position.
4. Clear the code and start the engine.

• If the misfire moves to cylinder 2, the coil is defective – replace it.
• If the misfire stays on cylinder 4, the coil is likely fine; continue testing.

Tools: 10 mm socket, OBD‑II scanner, torque wrench (if coil bolts are torque‑spec’d).

5. Fuel Injector Function Check

Audible Test:

• With the engine idling, place a mechanic’s stethoscope on the injector’s fuel rail near cylinder 4. You should hear a consistent “click” each time the injector fires.

Electrical Test (if scanner supports):

• Use the “Injector Pulse Width” live data PID. Compare cylinder 4’s pulse width to the others.

• Good: Pulse width within 5 % of neighboring cylinders.

• Bad: Significantly shorter pulse or no click – the injector may be clogged or electrically failed.

Repair: Clean the injector with a professional cleaning kit or replace it. Typical aftermarket injector price: $30‑$70 per unit.

6. Compression Test

1. Remove all spark plugs.
2. Screw the compression gauge into cylinder 4’s plug hole.
3. Crank the engine for 4‑5 revolutions. Record the pressure.
4. Repeat for cylinders 1‑3 for comparison.

• Good: Cylinder 4 pressure within 10‑15 % of the highest reading.
• Bad: Pressure significantly lower → possible worn rings, valve seat damage, or head‑gasket leak.

Repair: Low compression often requires a machine shop or engine rebuild. If you suspect a blown head gasket, look for coolant in the oil or white smoke from the exhaust.

7. Vacuum Leak Detection

1. With the engine at idle, spray a small amount of carburetor cleaner around the intake manifold, especially near the runner feeding cylinder 4.
2. Listen for a change in RPM. A rise indicates the engine is drawing in extra air (leak).

• Good: No RPM change.
• Bad: RPM spikes – locate the leak (cracked hose, loose clamp) and replace the offending part.

8. Final Verification

After any repair, clear all codes, disconnect the battery for 30 seconds to reset the ECU, then reconnect and start the engine. Drive the car through a normal city‑stop cycle and a brief highway acceleration. Re‑scan; the P0304 should be gone, and no new misfire codes should appear.

Cost Breakdown – DIY vs. Shop

Total DIY estimate (average scenario – plug + coil + injector): $70‑$150 in parts plus roughly 2 hours of labor, well under $300 total.

Typical shop estimate: $250‑$500 for the same combo, depending on labor rates.

Dealership: Often higher due to OEM parts markup and diagnostic fees; expect $400‑$800 for a full misfire repair package.

Preventive Maintenance Tips

• Replace spark plugs every 30,000–50,000 mi (or per manufacturer interval).
• Use fuel‑grade gasoline that meets the vehicle’s octane rating; low‑octane fuel can cause knock and misfires.
• Keep the fuel system clean – periodic fuel injector cleaning can prevent deposits that starve a single cylinder.
• Inspect ignition coils every 60,000 mi; many modern coils are “lifetime” but can still fail prematurely.
• Monitor engine coolant – overheating can warp the head and lead to compression loss.

Following these steps reduces the likelihood of a repeat P0304.

Can You Still Drive With This Code?

Short answer: Yes, you can drive a short distance at light throttle, but avoid hard acceleration, towing, or high‑rpm highway driving. Prolonged operation can overheat the catalytic converter and cause costly damage.

A persistent misfire will almost certainly cause a failed emissions test because unburned fuel will raise the hydrocarbon (HC) reading. Fix the issue before your next state inspection.

Frequently Asked Questions

Can I drive my car with a P0304 code? You may limp to a safe location, but you should not keep the car on the road for more than a few miles, especially under load.

How long can I drive with a P0304 code? Only until the check‑engine light flashes (indicating a severe misfire). At that point, shut the engine down to protect the catalytic converter.

How do I fix code P0304? Follow the step‑by‑step guide above: start with a visual inspection, then check freeze‑frame data, spark plug, coil, injector, compression, and finally vacuum leaks.

Will a P0304 cause a failed emissions test? Yes. The misfire sends raw fuel into the exhaust, raising hydrocarbon levels and likely failing the test.

What is the typical cost to fix a cylinder 4 misfire? If it’s just a plug and coil, expect $100‑$250 total. More serious internal damage can exceed $1,000.

Related DTCs You Might Encounter

• P0303 – Cylinder 3 Misfire Detected – often shares the same root causes as P0304.
• P0300 – Random/Multiple Cylinder Misfire Detected – indicates a broader issue affecting several cylinders.

Review these pages if your scanner shows multiple misfire codes at once.

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