P0304 means your engine's computer has identified consistent misfires in cylinder 4. On most inline-4 engines, cylinder 4 is at the rear near the firewall, which can make access slightly harder. The fault is isolated to that cylinder's ignition, fuel, or compression system.
What does P0304 mean?
P0304 means your engine's computer has identified consistent misfires in cylinder 4. On most inline-4 engines, cylinder 4 is at the rear near the firewall, which can make access slightly harder. The fault is isolated to that cylinder's ignition, fuel, or compression system.
P0304 is a misfire-related diagnostic trouble code classified as high severity. When your vehicle's ECU detects this condition, it stores P0304 and illuminates the check engine light.
What are the symptoms of P0304?
If your vehicle has triggered P0304, you may notice one or more of these symptoms:
Check engine light steady or flashing
Rough idle with a noticeable miss
Vibration through the vehicle body at idle
Loss of power when accelerating
Increased fuel consumption
Raw fuel smell from the tailpipe
What causes P0304?
Here are the most common causes of P0304, ranked by how likely they are to be the culprit:
highFailed ignition coil on cylinder 4
highWorn or fouled spark plug on cylinder 4
mediumClogged or electrically failed fuel injector on cylinder 4
mediumIntake manifold gasket leak near cylinder 4
lowLow compression from burnt exhaust valve or worn rings on cylinder 4
Is it safe to drive with P0304?
No - address this immediately. A persistent misfire on cylinder 4 will damage the catalytic converter and may cause sudden power loss. Do not ignore a flashing check engine light. Have the vehicle diagnosed as soon as possible.
How do you diagnose P0304?
Follow these steps to pinpoint the root cause of P0304:
Retrieve freeze-frame data and note any companion misfire codes (P0300-P0303) that may indicate a wider problem
Swap the ignition coil from cylinder 4 with cylinder 1 or 2, clear codes, and test drive. If the misfire code follows the coil, replace it
Remove and inspect the spark plug on cylinder 4. On inline-4 engines, cylinder 4 is nearest the firewall, so a spark plug socket with an extension may be needed
Verify the fuel injector on cylinder 4 is receiving its electrical pulse using a noid light. Also check for visible fuel leaks at the injector seal
If ignition and fuel are ruled out, perform a compression and leak-down test on cylinder 4 to check for valve seating issues or ring wear
How much does P0304 cost to fix?
Repair costs for P0304 vary depending on the root cause and your vehicle. Here are typical estimates:
Repair
Parts Cost
Labor Cost
Total Estimate
DIY Difficulty
Replace spark plug (cylinder 4)
€5–€25
€30–€100
€35–€125
Easy
Replace ignition coil (cylinder 4)
€25–€130
€30–€110
€55–€240
Easy
Replace fuel injector (cylinder 4)
€50–€200
€70–€220
€120–€420
Moderate
Valve or compression repair
€200–€700
€400–€1400
€600–€2100
Professional
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
P0304 on specific vehicles
P0304 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:
P0304 on Ford Focus
Ford Focus • 2005-2022
On the 1.6 Ti-VCT and 2.0 Duratec engines, cylinder 4 is at the rear of the engine near the firewall. The 1.0 EcoBoost 3-cylinder does not have a cylinder 4. The 2.0 GDI engine can develop carbon buildup affecting cylinder 4.
What causes P0304 on a Ford Focus?
Ford FocusCarbon buildup on 2.0 GDI intake valves
Ford FocusIgnition coil failure on 1.6 Ti-VCT, especially at higher mileage
P0304 on Skoda Octavia
Skoda Octavia • 2005-2024
The 1.8 TSI and 2.0 TSI engines in the Octavia vRS share the EA888 platform with the VW Golf GTI. Cylinder 4 on these engines is at the gearbox end and benefits from the same walnut blasting maintenance to prevent carbon misfire issues.
What causes P0304 on a Skoda Octavia?
Skoda OctaviaCarbon buildup on direct-injection intake valves (1.8/2.0 TSI)
Skoda OctaviaIgnition coil degradation on 1.6 MPI
P0304 repair cost for Skoda Octavia
Estimated costs for Skoda Octavia ownersadjusted higher than average due to Skoda-specific parts pricing
Repair
Parts Cost
Labor Cost
Total Estimate
DIY Difficulty
Replace spark plug (cylinder 4)
€5–€26
€32–€105
€37–€131
Easy
Replace ignition coil (cylinder 4)
€26–€137
€32–€116
€58–€252
Easy
Replace fuel injector (cylinder 4)
€53–€210
€74–€231
€126–€441
Moderate
Valve or compression repair
€210–€735
€420–€1470
€630–€2205
Professional
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
P0304 on Mercedes C-Class
Mercedes-Benz C-Class • 2007-2024
The M274 2.0-litre turbo engine in the C200/C250 (W205) has documented issues with ignition coil failures and carbon buildup. Cylinder 4 access is restricted on the W205 due to tight engine bay packaging. Mercedes recommends premium plugs replaced at 60,000 km.
What causes P0304 on a Mercedes C-Class?
Mercedes C-ClassIgnition coil failure on M274 turbo engine
Mercedes C-ClassCarbon buildup on direct-injection intake valves
P0304 repair cost for Mercedes C-Class
Estimated costs for Mercedes C-Class ownersadjusted higher than average due to Mercedes-Benz-specific parts pricing
Repair
Parts Cost
Labor Cost
Total Estimate
DIY Difficulty
Replace spark plug (cylinder 4)
€8–€38
€45–€150
€53–€188
Easy
Replace ignition coil (cylinder 4)
€38–€195
€45–€165
€83–€360
Easy
Replace fuel injector (cylinder 4)
€75–€300
€105–€330
€180–€630
Moderate
Valve or compression repair
€300–€1050
€600–€2100
€900–€3150
Professional
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
P0304 on SEAT Leon
SEAT Leon • 2005-2024
The Leon shares VW Group EA211 and EA888 engines. The 1.4 TSI (EA211) in Mk3 models can develop ignition coil failures on any cylinder. Cylinder 4 misfires on the 2.0 TSI Cupra are often traced to carbon buildup from high-performance driving.
What causes P0304 on a SEAT Leon?
SEAT LeonIgnition coil failure on 1.4 TSI (EA211)
SEAT LeonCarbon buildup on 2.0 TSI Cupra under hard driving
P0304 repair cost for SEAT Leon
Estimated costs for SEAT Leon ownersadjusted higher than average due to SEAT-specific parts pricing
Repair
Parts Cost
Labor Cost
Total Estimate
DIY Difficulty
Replace spark plug (cylinder 4)
€5–€26
€32–€105
€37–€131
Easy
Replace ignition coil (cylinder 4)
€26–€137
€32–€116
€58–€252
Easy
Replace fuel injector (cylinder 4)
€53–€210
€74–€231
€126–€441
Moderate
Valve or compression repair
€210–€735
€420–€1470
€630–€2205
Professional
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
Related diagnostic codes
These codes are often seen alongside P0304 or indicate related issues:
P0304 tells you that the engine control module has detected repeated misfires on cylinder 4. The ECU counts misfires by monitoring tiny variations in crankshaft speed and flags the code when cylinder 4 misfires exceed the allowed threshold.
Is cylinder 4 hard to access?
On inline-4 engines, cylinder 4 is typically closest to the firewall at the rear of the engine bay. Access can be tighter, especially for spark plugs and coils. On some vehicles you may need to remove an engine cover or intake components.
Can a P0304 misfire damage my engine?
The misfire itself does not usually damage the engine block, but unburned fuel washing the cylinder walls can dilute oil and accelerate ring wear over time. The bigger immediate risk is catalytic converter damage from overheating.
Why did P0304 appear after changing spark plugs?
This is usually caused by an incorrectly gapped plug, the wrong plug type for your engine, a plug not seated properly, or a damaged ignition coil boot. Remove the cylinder 4 plug, verify the gap and part number, and re-seat it.
Should I replace all ignition coils if only cylinder 4 is misfiring?
It is not strictly necessary, but if the vehicle has high mileage (over 100,000 km) and the other coils are original, replacing the full set is a good preventive measure. Coils of the same age tend to fail in sequence.
