P0304 on Kia
Cylinder 4 Misfire Detected
P0304 on Kia vehicles indicates cylinder 4 misfire detected. Au7o has documented this code across 4 Kia models — most commonly on K5, Rio, Spectra. 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 Kia range from $80 to $2,500, 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 Kia by Model
Kia K5(1 issue)
- SmartStream G2.5 GDI Engine Piston Ring/Oil Consumption Issue2021-2024
Owners of K5 models equipped with the 2.5L naturally aspirated GDI engine (SmartStream G2.5) have reported excessive oil consumption between change intervals, sometimes consuming over a quart per 1,000 miles. The root cause is linked to piston ring sealing tolerances that allow oil to enter the combustion chamber, producing blue smoke from the exhaust on cold starts and acceleration. NHTSA complaint database reflects multiple owners reporting this on 2021-2023 model years, with Kia issuing TSB guidance for inspection and potential short block replacement.
Kia Rio(3 issues)
- Fuel Injector Failure and Engine Stalling2012-2017
The 1.6L GDI engine in the 2012-2017 Rio can experience fuel injector failures that cause rough idle, misfires, and stalling. The GDI injector tips develop carbon deposits and the injector seals deteriorate, leading to poor spray patterns and fuel delivery issues. In some cases, injectors leak fuel onto the engine, creating a fire risk that led to a recall.
- Timing Belt Neglect Leading to Bent Valves and Sudden No-Start on 1.6L Engines2001-2005
Early Rio models with the 1.6L DOHC engine are widely documented as suffering severe engine damage when the timing belt breaks or jumps. Because this engine is treated by owners and repair databases as an interference design in real-world failures, a belt failure can cause piston-to-valve contact, leaving the car with a sudden stall or crank-no-start and expensive cylinder head repairs. This is one of the most commonly discussed catastrophic failures on first-generation Rio models.
- Ignition Coil and Plug Wire Failure Causing Misfire, Hesitation, and Check Engine Light2001-2011
Across both generations of Rio, owners frequently report ignition-related misfires caused by failing coil packs, deteriorated plug wires on earlier cars, or worn spark plugs. The problem typically shows up as rough idle, hesitation under load, flashing MIL, and poor fuel economy. It is common enough in owner complaints and repair forums to be considered a recurring Rio issue, especially as these cars age.
Kia Spectra(1 issue)
- Ignition Coil Pack Failure Causing Misfire2000-2009
Ignition coil packs (coil-on-plug design on later models, distributor coil on earlier) fail causing engine misfires, rough running, and poor fuel economy. Heat cycling weakens the coil insulation leading to internal shorts.
Kia Stinger(1 issue)
- Intake Valve Carbon Buildup (GDI Engines)2018-2023
All turbocharged GDI engines in the Stinger (3.3T, 2.5T, 2.0T) develop carbon deposits on the intake valves over time because GDI fuel injection sprays fuel directly into the combustion chamber rather than onto the valve stems. Without port injection washing the valves, carbon accumulates and restricts airflow, causing rough idle, misfires, and power loss. This typically becomes noticeable between 40,000-80,000 miles.
Looking for P0304 on a different make?
View P0304 across all makes →Frequently Asked Questions
What does P0304 mean on Kia?▼
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 Kia specifically, this code is documented across 4 models.
What causes P0304 on Kia vehicles?▼
Common causes on Kia: 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 Kia?▼
Repair costs on Kia range from $80 to $2,500, depending on the specific model and root cause.
Which Kia models have P0304 documented?▼
Au7o has documented P0304 on 4 Kia models: K5, Rio, Spectra, Stinger.