P0304 Code: Cylinder 4 Misfire Detected
If your scanner returned the P0304 code, your engine control module has identified a misfire specifically in cylinder 4. Just like P0303 points to cylinder 3, the P0304 code gives you a precise starting point — you don’t have to guess which cylinder is the problem.
A misfire in cylinder 4 means the air-fuel mixture in that cylinder isn’t igniting correctly. The engine drops a power stroke, and you feel it through the steering wheel, the seat, and the throttle response. The cause is almost always the spark, the fuel, or the compression — and a systematic approach gets you to the answer quickly.
What You’ll Notice
- Check engine light on — may be flashing in severe cases
- Rough, uneven idle
- Engine vibration or shaking
- Hesitation or stumble when accelerating
- Noticeable loss of power
- Fuel economy worse than usual
- Smell of unburned fuel from the exhaust in severe cases
If the check engine light is flashing, the P0304 code is active right now and the misfire is severe enough to damage the catalytic converter in real time. Don’t drive any further than necessary.
Where Is Cylinder 4?
- Inline 4-cylinder: Cylinder 4 is at the rear of the engine, furthest from the accessory belt
- Inline 6-cylinder: Fourth from the accessory belt end
- V6 / V8 engines: Location varies — check your vehicle’s service manual to confirm
On inline 4-cylinder engines, cylinder 4 is at the back — which can make access slightly more awkward than the front cylinders. Worth knowing before you start.
What Usually Causes the P0304 Code
1. Worn or fouled spark plug The most common cause of the P0304 code. A plug past its service life produces a weak, inconsistent spark. A plug fouled with oil or carbon doesn’t fire reliably at all. This is always the first thing to check — it’s cheap, quick, and fixes P0304 more often than anything else.
2. Failed ignition coil on cylinder 4 Each cylinder on a modern coil-on-plug engine has its own coil. When cylinder 4’s coil starts to fail, the spark weakens and the misfire begins. The coil swap test — the same method used to diagnose P0301, P0302, and P0303 — tells you in minutes whether the coil is the problem.
3. Faulty fuel injector on cylinder 4 A clogged injector can’t deliver the right amount of fuel to cylinder 4. A dead injector causes a complete misfire every cycle. If the plug and coil are both fine, the injector is next on the list.
4. Low compression in cylinder 4 A burnt exhaust valve, worn piston rings, or a head gasket issue affecting cylinder 4 drops compression below what combustion requires. If this is the cause, replacing the plug or coil won’t fix the P0304 code — the misfire returns immediately.
5. Vacuum leak near cylinder 4 A cracked intake manifold gasket or split vacuum hose near cylinder 4 allows unmetered air in, leaning out the mixture in that cylinder specifically. Typically worse at idle and improves at higher RPM.
6. Wiring fault to cylinder 4 A damaged wire or corroded connector on the coil or injector circuit for cylinder 4 can cause an intermittent or permanent P0304. Less common, but worth checking if the mechanical components all test fine.
How Serious Is the P0304 Code?
Serious — address it promptly.
Every misfire cycle sends unburned fuel into the exhaust system, where it burns inside the catalytic converter. Repeated misfires overheat and destroy the catalyst material. Catalytic converter replacement costs significantly more than the ignition parts that typically cause the P0304 code.
On top of that, an engine running on fewer cylinders than designed puts extra load on the remaining components. Catch it early and the fix is usually straightforward and inexpensive.
How to Diagnose the P0304 Code
Step 1 — Confirm the P0304 code and read all stored codes
Scan everything, not just P0304. Check whether P0300 or other cylinder-specific codes are present. Multiple codes together change the diagnostic picture — a single P0304 with nothing else means the problem is genuinely isolated to cylinder 4.
Step 2 — Pull the cylinder 4 spark plug
Remove it and compare it side by side with plugs from the other cylinders. A plug that looks different — fouled, oil-coated, blistered, or heavily worn — confirms something specific is happening in cylinder 4. Replace it and retest before going any further.
Step 3 — Swap the ignition coil
Move cylinder 4’s coil to another cylinder — cylinder 2 works well. Clear all codes and drive. If the misfire code shifts to P0302, the coil is causing the P0304 code. If P0304 stays on cylinder 4, the coil is fine and you move on.
Step 4 — Test the fuel injector
Listen to cylinder 4’s injector with a mechanic’s stethoscope while the engine runs. A healthy injector produces a steady rhythmic click. Test resistance with a multimeter and compare to the manufacturer’s specification. A dead or out-of-spec injector on cylinder 4 confirms the fuel delivery fault.
Step 5 — Check for vacuum leaks near cylinder 4
With the engine idling, carefully spray carburetor cleaner around the intake manifold gasket and vacuum lines near cylinder 4. A change in idle at a specific spot pinpoints the leak. A smoke test is the more thorough option if you want to be certain.
Step 6 — Compression test
If plugs, coil, and injector all check out, test cylinder 4’s compression. A reading significantly lower than the other cylinders — or below the manufacturer’s minimum specification — means an internal engine fault is behind the P0304 code.
How to Fix the P0304 Code
Replace the spark plug — Start here every time. Replace the cylinder 4 plug and inspect the others while you’re at it. On a high-mileage engine with original plugs, replace the full set.
Replace the ignition coil — If the coil swap test confirmed a faulty coil, replace it. Don’t swap it back — fit a new one.
Clean or replace the fuel injector — A quality fuel system cleaner handles mild clogging. A confirmed failed injector needs to be replaced.
Repair vacuum leaks — Replace cracked hoses and failed gaskets. The intake manifold gasket requires more labor but the part itself is inexpensive.
Engine repair — If low compression is causing the P0304 code, the repair depends on the specific cause. A burnt valve means head work. Worn rings require more extensive engine attention. Get a proper diagnosis before committing to any internal engine repair.
What It Might Cost to Fix the P0304 Code
| Repair | Estimated Cost |
|---|---|
| Spark plug replacement | $20 – $150 |
| Ignition coil replacement | $80 – $300 |
| Fuel injector cleaning | $50 – $150 |
| Fuel injector replacement | $150 – $400 |
| Vacuum leak repair | $100 – $400 |
| Head work / engine repair | $500 – $2,500+ |
Costs vary by vehicle make, model, and labor rates in your area.
Related Codes
- P0300 — Random/Multiple Cylinder Misfire Detected
- P0301 — Cylinder 1 Misfire Detected
- P0302 — Cylinder 2 Misfire Detected
- P0303 — Cylinder 3 Misfire Detected
- P0305 — Cylinder 5 Misfire Detected
- P0354 — Ignition Coil D Primary/Secondary Circuit Malfunction
- P0204 — Injector Circuit Malfunction Cylinder 4
Frequently Asked Questions
Can I drive with the P0304 code? With a steady light and no severe symptoms, short necessary trips are possible. But every kilometre driven on a misfire damages the catalytic converter. If the light is flashing, minimize driving immediately and get it seen to.
How do I know if the P0304 code is the coil or the plug? Replace the plug first — it’s cheaper and more often the cause. If the P0304 code returns, do the coil swap test. Move cylinder 4’s coil to another cylinder and see if the misfire follows it. Ten minutes and you have a definitive answer.
Can P0304 and P0300 appear at the same time? Yes, and it’s common. P0300 flags widespread misfires across the engine. P0304 identifies cylinder 4 as the main source. Both together usually means cylinder 4 is the primary problem but the misfire is severe enough to affect the overall engine reading too.
I replaced the plug and coil but P0304 is still there. What next? Test the fuel injector — resistance check, click test, and ideally a flow test if a shop has the equipment. After that, a compression test on cylinder 4. At that point you’ve eliminated the three most common causes and the fault is either a vacuum leak or an internal engine problem.
How long can I leave the P0304 code before it becomes a bigger problem? There’s no safe window. A flashing light means damage is happening right now. A steady light means you have some time — but not indefinitely. Fix the P0304 code at the first opportunity before a cheap repair becomes an expensive one.
This guide is for informational purposes only. For an accurate diagnosis, always consult a qualified mechanic.
