P0304 on Alfa Romeo
Cylinder 4 Misfire Detected
P0304 on Alfa Romeo vehicles indicates cylinder 4 misfire detected. Au7o has documented this code across 1 Alfa Romeo model — most commonly on Stelvio. 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 Alfa Romeo range from $500 to $1,200, 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 Alfa Romeo by Model
Alfa Romeo Stelvio(1 issue)
- 2.0L Turbo Intake Valve Carbon Buildup Causing Cold-Start Misfire2018-2026
The 2.0L GME turbo four is direct injected, so no fuel washes over the back of the intake valves. Crankcase vapour routed through the PCV system bakes onto the valve heads and forms hard carbon deposits that disrupt airflow and seal quality. What makes the Stelvio case notable is how early it appears: Stelvio and Giulia forum members report being sent for walnut blasting at Alfa's direction at roughly 6,000-6,500 miles because of cold-start misfires, an order of magnitude sooner than the 75,000-125,000 mile window typical of other direct-injection engines. Symptoms are worst on a cold start and often clear once the engine warms, which is why the condition is frequently dismissed on a test drive. This is a 2.0L issue and should not be merged with the 2.9L V6 Quadrifoglio's separate intake-side story.
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What does P0304 mean on Alfa Romeo?▼
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 Alfa Romeo specifically, this code is documented across 1 model.
What causes P0304 on Alfa Romeo vehicles?▼
Common causes on Alfa Romeo: 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 Alfa Romeo?▼
Repair costs on Alfa Romeo range from $500 to $1,200, depending on the specific model and root cause.
Which Alfa Romeo models have P0304 documented?▼
Au7o has documented P0304 on 1 Alfa Romeo model: Stelvio.