Cylinder 4 Misfire Detected
In short
P0304 means the engine computer detected that cylinder 4 is misfiring — it isn't burning its air/fuel mixture properly on every combustion cycle. The single most common real-world cause is a worn spark plug or a failing ignition coil on that specific cylinder. Because the fault is isolated to one cylinder, it's usually traceable and affordable to fix.
Is it safe to drive with P0304?
A single cylinder misfire is generally safe to drive a short distance, but you should fix it promptly. A persistent misfire dumps raw fuel into the exhaust, which can overheat and permanently damage the catalytic converter. If the check engine light is flashing, stop driving and have it diagnosed immediately — flashing indicates an active misfire actively harming the cat.
Symptoms
- Check engine light on, or flashing during the misfire
- Rough or shaky idle, especially when stopped at lights
- Noticeable hesitation or stumble under acceleration
- Loss of power and reduced fuel economy
- Engine vibration felt through the steering wheel or seat
- Occasional popping or sputtering from the exhaust and a fuel smell
Common causes (most → least likely)
How to diagnose it (before buying parts)
- 1 Scan for all stored codes — note any companion codes (lean/rich, fuel trim, or multiple-cylinder misfire codes) that point to a broader root cause rather than a single-cylinder part.
- 2 Use a scan tool's live data to view misfire counts per cylinder and confirm the misfires are isolated to cylinder 4 versus spread across cylinders.
- 3 Swap the cylinder 4 ignition coil with a coil from a known-good cylinder, clear codes, and recheck — if the misfire follows the coil to the new cylinder, the coil is the culprit.
- 4 Remove and inspect the cylinder 4 spark plug for wear, fouling, oil, cracked porcelain, or incorrect gap, and compare it to the other plugs.
- 5 If ignition checks out, perform a compression (and ideally cylinder leak-down) test on cylinder 4 to rule out a mechanical fault before replacing fuel or other parts.
Repair options & cost
By manufacturer
Coil-on-plug failures are a common single-cylinder misfire cause; iridium plugs are long-life, so a failed coil is often the real issue when one cylinder misfires.
Coil-on-plug units and worn plugs are frequent misfire sources; some EcoBoost engines are sensitive to extended plug intervals, causing cylinder-specific misfires.
Individual ignition coils can fail and cause an isolated cylinder misfire; check for water intrusion in the coil wells on some models.
Coil pack failures across TSI/TFSI engines are well-documented and often cause single-cylinder misfires; replacing coils as a set is common practice.
Frequently asked questions
Can I drive with a P0304 code?
For a short distance, usually yes, but you should fix it soon. A sustained misfire can overheat and ruin the catalytic converter. If the check engine light is flashing, pull over and avoid driving — that signals active damage.
How much does it cost to fix P0304?
Most P0304 repairs are inexpensive because the fault is isolated to one cylinder. A spark plug or coil replacement typically runs 60-400 dollars at a shop, or much less DIY. Mechanical causes like low compression are rare but far more expensive.
Will a misfire clear itself?
No — the underlying cause must be repaired. The code may stop reappearing temporarily if the misfire is intermittent, but it will return until the worn part or mechanical issue is fixed.
Which cylinder is cylinder 4?
Cylinder 4 is identified by your engine's specific firing layout, not always the fourth plug from one end. Check a cylinder-numbering diagram for your exact make, model, and engine to locate it correctly before swapping parts.