The P0304 diagnostic trouble code on your 2021 Honda HR-V indicates the engine control module has detected consistent misfires in cylinder 4. The first generation HR-V uses a reliable 1.8L four-cylinder engine.
1.8L Engine Layout
The 2021 HR-V uses the R18Z1 1.8L i-VTEC four-cylinder producing 141 horsepower. This naturally aspirated engine uses port fuel injection. Cylinder 4 is the rearmost cylinder on this inline-4.
Common Causes
- Ignition Coil Failure: Individual coil-on-plug units can fail
- Spark Plug Wear: Iridium plugs eventually need replacement
- Fuel Injector Problems: Port injection system issues
- Vacuum Leaks: Intake manifold or gasket failures
- Valve Adjustment: May need clearance check
Diagnostic Approach
Use a Honda-compatible scanner for detailed diagnostics. The port injection system is simpler than direct injection. Monitor misfire counts. Test ignition components systematically.
Repair Solutions
Replace failed ignition coils with Honda OEM or quality aftermarket parts. Install properly specified spark plugs. Clean fuel injectors as needed. Repair vacuum leaks.