P0304 on Mercedes-Benz: Causes, Symptoms and Fix Cost
Quick Answer
Not safe to drive
P0304 on a Mercedes-Benz means your engine's computer has identified consistent misfires in cylinder 4. Where cylinder 4 sits depends on how the engine is mounted: on a lengthwise (longitudinal) inline-4, usual in rear-wheel-drive cars, it is the cylinder nearest the firewall, while on a transverse engine, usual in front-wheel-drive cars, it is at the gearbox end. The fault is isolated to that cylinder's ignition, fuel, or compression system.
- Severity
- High · Misfire
- Typical repair cost, all makes
- €35-€2100
Connect a standard OBD2 adapter (Bluetooth LE on iPhone) to your Mercedes-Benz and Skanyx tells you what P0304 means on that engine, whether it is safe to keep driving, and what the fix should cost. Free on iPhone and Android.
Get Skanyx freeWhat does P0304 mean on a Mercedes-Benz?
The Quick Answer above is the generic definition. What follows is the model-specific detail for the Mercedes-Benz models that log P0304.
Mercedes 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. Because the labour to reach that cylinder is the expensive part, it is worth changing plugs at the interval in the car's service schedule rather than waiting for a misfire.
What causes P0304 on a Mercedes-Benz?
Beyond the generic causes listed on the main P0304 page, these are the patterns noted for Mercedes-Benz models:
Ignition coil failure on M274 turbo engine
: Mercedes C-Class
Carbon buildup on direct-injection intake valves
: Mercedes C-Class
How to diagnose P0304 on a Mercedes-Benz with OBD2
- 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 a longitudinal inline-4 it is nearest the firewall, on a transverse one it is at the gearbox end, and 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 on a Mercedes-Benz? (EUR)
Estimated repair costs for P0304 in EUR, compiled across makes. Expect Mercedes-Benz parts and labour towards the upper end of each range.
| 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 October 2026. Costs vary by region, vehicle, and shop.
Codes related to P0304
These codes are often seen alongside P0304 or point to related issues, on any make:
Looking for the full P0304 reference (all makes, full diagnosis flow, complete repair cost matrix)? See the main P0304 guide