P0304 Honda Civic
Cylinder 4 Misfire Detected · Powertrain Code · High · Repair: Moderate
What is P0304?
Misfire detected in cylinder 4. Typically caused by a worn spark plug, bad ignition coil, faulty fuel injector, or low compression in that cylinder.
When the Honda Civic triggers code P0304 (cylinder 4 misfire detected), it typically points to an issue in the powertrain system. This is a high severity code, so address it promptly to avoid further damage or safety concerns. Below you'll find the most common causes, symptoms, and repair options specific to the Honda Civic.
This repair is rated moderate, meaning some experience and specialty tools required.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your Honda Civic.
Common Causes on Honda Civic
- Worn spark plug on cylinder 4
- Faulty ignition coil on cylinder 4
- Carbon buildup on intake valves (Earth Dreams DI engines)
- Low compression from worn piston rings or valve seats
- Vacuum leak at cylinder 4 intake runner
Symptoms
- Flashing CEL during active misfire
- Rough idle with engine vibration
- Noticeable loss of power
- Poor acceleration
- Uneven exhaust note
Notes for Honda Civic
If misfires appear on multiple cylinders over time, suspect carbon buildup on DI engines rather than individual coil failures.