Can all four misfire codes appear at once?

No, P0304 targets cylinder 4 only. However, if multiple cylinders are misfiring, you'd see multiple codes (P0301, P0302, P0303, and P0304) or P0300 (random/multiple misfire). If only P0304 is present, the fault is isolated to cylinder 4.

Is a misfire on cylinder 4 worse than cylinder 1?

No. The severity is the same regardless of which cylinder misfires. Any single-cylinder misfire will damage your catalytic converter over time. The diagnosis and fix are identical.

My car has been sitting for six months — could that cause P0304?

Yes. Old fuel loses octane, and moisture can form in the tank. The first few tank-fulls of fresh fuel might clear it. Bad battery voltage during storage can also affect coil performance. But if the code persists after fresh fuel and a full charge, move to spark plug and coil inspection.

Should I replace all four spark plugs if one is bad?

Spark plugs wear together, so if cylinder 4's plug is worn, the others likely are too. It's sensible to replace all four at once — costs about £80–£120 for parts and labour — rather than replace them one at a time. Coil packs are different; only replace the faulty one unless you suspect others are failing.

P0304 — Cylinder 4 Misfire Detected

What Is P0304?

P0304 is the diagnostic trouble code for misfires on cylinder 4. Your engine has four, six, or more cylinders depending on the vehicle, and your ECU monitors combustion in each one. When cylinder 4 misfires — meaning the spark plug doesn't fire the fuel, or the fuel charge doesn't burn — the ECU counts the event. After a certain number of misfires are recorded per driving cycle (typically 5–10 misfire events per 1000 revolutions), the code is stored and the check engine light illuminates.

The most dangerous consequence is catalytic converter damage. Unburnt fuel flows into the exhaust and ignites inside the cat, causing extreme heat and permanent damage. A replacement cat can cost £900–£1500.

Common Symptoms

Engine running roughly at idle, feeling lumpy or unstable
Hesitation, stumbling, or loss of power during acceleration
Noticeable vibration in the steering wheel, pedals, or seat
Check engine light displayed (solid or flashing)
Engine sounds like it's only running on three cylinders instead of four

Common Causes

Aged or Worn Spark PlugThe plug for cylinder 4 is at the end of its life, with excessive electrode wear or heavy carbon fouling preventing ignition.

Failed Ignition Coil PackThe coil generating spark for cylinder 4 has failed internally, producing no spark or a very weak spark.

Blocked Fuel InjectorCarbon deposits inside the cylinder 4 injector prevent it from delivering fuel or delivering it inconsistently.

Low Engine CompressionWorn piston rings, a stuck exhaust valve, or scored cylinder walls specific to cylinder 4 reduce combustion pressure.

Vacuum Leak on IntakeAn air leak near cylinder 4's intake manifold entry draws unmetered air, leaning the mixture and weakening spark.

Poor Quality or Contaminated FuelWater, sediment, or stale fuel in the tank prevents complete combustion, especially on specific cylinders.

How to Diagnose P0304

Inspect the Cylinder 4 Spark Plug

Locate cylinder 4 using your engine bay diagram or repair manual. Disconnect the coil pack or plug wire, then unscrew the spark plug. A healthy plug should have a slight tan or brown colour. If it's sooty black, covered in engine oil, or the centre electrode is completely worn away, the plug is faulty. Check the gap (typically 0.8–1.2 mm) and replace if needed.

Test the Cylinder 4 Coil

Remove the coil pack directly above cylinder 4. Look for cracks in the plastic, blackened terminals, or burn marks. If possible, swap it with the coil from another cylinder (such as cylinder 1), clear the code with an OBD scanner, and test drive. If the misfire code moves to the other cylinder, the coil was faulty. If it stays on cylinder 4, the issue lies elsewhere.

Check for Intake Vacuum Leaks

Start the engine and listen carefully for hissing sounds around the intake manifold, vacuum hoses, and gaskets. Spray soapy water around suspect areas — bubbles forming indicate a leak. Pay close attention to areas near cylinder 4's intake port. A vacuum leak allows extra air to enter the combustion chamber, causing misfires.

Verify Fuel Injector Function

Connect an OBD scanner capable of showing live fuel system data. Monitor the cylinder 4 fuel injector — a healthy injector shows consistent pulse timing and electrical commands. Listen with a stethoscope on the injector body; you should hear rapid ticking. No sound or irregular clicking suggests injector failure or blockage. Try a fuel injector cleaning additive; if unsuccessful, professional cleaning or replacement may be needed.

Perform a Compression Test on Cylinder 4

If ignition and fuel checks pass, suspect internal engine wear. Disconnect all spark plugs and fuel injectors to prevent accidental firing. Install a compression tester in the cylinder 4 spark plug hole and crank the engine 6–10 times. Healthy compression is typically 130–150 psi. If cylinder 4 reads 20+ psi lower than the other cylinders, it has worn piston rings, a damaged valve, or cylinder wall damage.

Exhaust System Risk Misfiring sends unburnt fuel into the exhaust system where it ignites, overheating the catalytic converter. Do not delay diagnosis. If your check engine light is flashing (severe misfire), stop driving immediately — multiple misfires per cycle indicate a serious problem.

Mechanic's Corner — P0304 on UK Cars

On inline-4 engines, cylinder 4 (the last cylinder in the bank) shares some thermal and coolant characteristics with cylinder 1 at the other end. On engines with a known head gasket weakness — Ford 1.6 TDCi, Vauxhall 1.6 CDTI, and some Peugeot/Citroen units — the end cylinders are often the first to show sealing issues, making P0301 and P0304 the two codes most likely to indicate early head gasket failure rather than ignition faults.

On diesel engines, P0304 (where supported) often points to an injector issue rather than ignition — diesels ignite by compression, not spark. A defective injector that either dribbles fuel into cylinder 4 at rest or fails to atomise fuel correctly under load produces the mechanical rough-running equivalent of a petrol misfire. A diesel injector return flow test will quantify how much fuel each injector returns at idle and at load.

Verdict

P0304 is almost always caused by spark plug or coil pack failure — these account for 85% of single-cylinder misfires. Both are relatively cheap and straightforward to fix (£30–£250 depending on vehicle model). Fuel injector and vacuum leak issues are less common but still simple to diagnose. Compression problems are rare and indicate internal engine wear requiring more extensive repairs (£800+).