Summary
P0304 means the PCM detected repeated misfires in cylinder 4. The most common causes are a worn spark plug (35%), a failing ignition coil (30%), and a faulty fuel injector (15%). On inline 4-cylinder engines, cylinder 4 is the farthest from the timing belt/chain, often in a hotter location that accelerates coil degradation. Use the coil-swap test first: swap cylinder 4's coil with another, clear codes, and drive.
Severity: Moderate to High — misfires damage catalytic converters
Safe to drive: Not recommended
Repair cost: $20–$500 depending on cause
DIY difficulty: Easy to Moderate
What does P0304 mean?
The PCM detected that cylinder 4 is consistently misfiring. Like all cylinder-specific misfire codes, P0304 uses crankshaft speed variation to identify which cylinder isn't contributing combustion force at its expected point in the firing order.
On most inline 4-cylinder engines, cylinder 4 is at the end opposite the timing belt/chain. This position often runs slightly hotter and is closer to the firewall, which can accelerate component wear — particularly ignition coils that already see high temperatures.