P0304 on MINI
Cylinder 4 Misfire Detected
P0304 on MINI vehicles indicates cylinder 4 misfire detected. Au7o has documented this code across 4 MINI models — most commonly on Cooper, Cooper S, GP. This code means the engine computer detected a misfire in cylinder number 4 — that cylinder isn't igniting its air-fuel mixture fully or at all. The system detects this by monitoring small changes in crankshaft rotation speed as each cylinder fires. You may feel a rough idle, shaking, hesitation or reduced power, and a persistent misfire can dump unburned fuel into the exhaust and overheat or damage the catalytic converter. Since it's pinpointed to one cylinder, the problem typically lies in that cylinder's spark, fuel, or compression. Typical repair costs on MINI range from $300 to $3,000, depending on the specific model and root cause.
Common Causes of P0304
- •Worn or fouled spark plug in cylinder 4
- •Failed or weak ignition coil on cylinder 4
- •Faulty spark plug wire or boot (where applicable)
- •Clogged, stuck or leaking fuel injector for cylinder 4
- •Vacuum or intake air leak near that cylinder
- •Low compression (worn rings, burnt/leaking valve, head gasket)
- •Damaged wiring or connector at the coil or injector
P0304 on MINI by Model
MINI Cooper(1 issue)
- N12/N16 Timing Chain Tensioner Failure2007-2013
The R56 Cooper's N12 and N16 engines suffer from premature timing chain tensioner and guide rail failure. The plastic chain guides deteriorate, causing chain slack that leads to rough running, rattling on startup, and potential catastrophic engine damage if the chain jumps timing.
MINI Cooper S(2 issues)
- N14/N18 Turbo Engine Timing Chain Catastrophic Failure2007-2013
The R56 Cooper S N14 engine (2007-2010) is particularly notorious for timing chain tensioner failure, even more so than the naturally aspirated N12. The higher cylinder pressures from turbocharging accelerate chain stretch and guide rail wear. The revised N18 (2011-2013) improved but didn't fully resolve the issue.
- Direct Injection Carbon Buildup on Intake Valves2007-2025
All turbocharged direct-injection MINI engines (N14, N18, B46/B48) accumulate carbon deposits on intake valves since fuel is injected directly into the cylinder rather than washing over the valves. This reduces airflow and causes rough running.
MINI GP(1 issue)
- Carbon Buildup on Intake Valves (Direct Injection) Causing Misfires/Power Loss2013-2021
Direct-injection turbo engines in MINI GP models can accumulate carbon deposits on intake valves because fuel does not wash the valve backs. Over time this can cause rough idle, hesitation, misfires, and reduced performance, especially noticeable on cold starts. This is widely documented in MINI owner reports and service bulletins covering DI drivability complaints on N18/B48 platforms.
MINI John Cooper Works(1 issue)
- Intake Valve Carbon Buildup from Direct Injection2008-2024
All MINI JCW models with direct injection (both N14 and B48 engines) accumulate carbon deposits on the intake valves because fuel is injected directly into the cylinder rather than washing over the valves as in port injection engines. Over 30,000-50,000 miles, carbon buildup restricts airflow, causing rough idle, misfires, reduced power, and poor fuel economy. The N14 engine (2008-2012) is particularly susceptible due to its higher oil consumption contributing additional oil vapor to the intake tract.
Looking for P0304 on a different make?
View P0304 across all makes →Frequently Asked Questions
What does P0304 mean on MINI?▼
P0304 stands for "Cylinder 4 Misfire Detected." This code means the engine computer detected a misfire in cylinder number 4 — that cylinder isn't igniting its air-fuel mixture fully or at all. The system detects this by monitoring small changes in crankshaft rotation speed as each cylinder fires. You may feel a rough idle, shaking, hesitation or reduced power, and a persistent misfire can dump unburned fuel into the exhaust and overheat or damage the catalytic converter. Since it's pinpointed to one cylinder, the problem typically lies in that cylinder's spark, fuel, or compression. On MINI specifically, this code is documented across 4 models.
What causes P0304 on MINI vehicles?▼
Common causes on MINI: Worn or fouled spark plug in cylinder 4, Failed or weak ignition coil on cylinder 4, Faulty spark plug wire or boot (where applicable), Clogged, stuck or leaking fuel injector for cylinder 4, Vacuum or intake air leak near that cylinder. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0304 on a MINI?▼
Repair costs on MINI range from $300 to $3,000, depending on the specific model and root cause.
Which MINI models have P0304 documented?▼
Au7o has documented P0304 on 4 MINI models: Cooper, Cooper S, GP, John Cooper Works.