P0304 indicates that the engine control unit (ECU) has detected a misfire in cylinder 4. This is a common OBD-II trouble code that can affect performance and efficiency on Honda CR‑V models across generations.
In a Honda CR-V, P0304 points to a problem in cylinder 4 that prevents the spark plug from igniting the air‑fuel mix properly. This can stem from ignition hardware, fuel delivery, air leaks, or engine mechanics. The result is usually rough running, reduced power, and a check engine light. Diagnosing the exact cause requires a systematic inspection of ignition components, fuel system, and basic engine health.
What P0304 means
P0304 is a cylinder-specific misfire code. It signals that the ECU detected abnormal combustion in cylinder 4, which can be triggered by a fault in the ignition system, fuel delivery, air intake, or engine mechanics. It is often accompanied by other P030x codes if more than one cylinder is affected, or by a P0300 code indicating random/multiple misfires.
Common causes
The following are frequent sources of P0304 in a Honda CR‑V. Identifying the root cause usually starts with hardware tests and a quick swap/inspection of known failure points.
Ignition issues
The ignition system is a common failure point for cylinder misfires. Consider inspecting or testing these components:
- Spark plug in cylinder 4 (wear, fouling, improper gap)
- Ignition coil or coil-on-plug for cylinder 4 (weak spark)
- Ignition coil wiring or connectors (corrosion or looseness)
- Ignition control module or related wiring (if applicable to your engine variant)
Issues in the ignition path are often the quickest to diagnose and fix, and addressing them can resolve the misfire without further complications.
Fuel delivery and air intake issues
Problems with fuel delivery or air supply can also trigger a cylinder misfire. Common items to check include:
- Clogged or leaking fuel injector for cylinder 4
- Low or inconsistent fuel pressure
- Vacuum leaks or intake manifold leaks near cylinder 4
- Mass airflow sensor or other air metering issues affecting cylinder 4’s air/fuel mixture
Fuel/air problems may require cleaning, replacement, or system pressure tests to verify proper operation.
Symptoms you might notice
Recognizing the symptoms can help you decide whether to pursue a DIY check or seek professional service. Common signs of P0304 include:
- Rough idle or engine vibration at low RPM
- Loss of power or hesitation during acceleration
- Increased fuel consumption or poor driveability
- Check Engine Light illuminated with code P0304 (and possibly related codes)
- Engine stalling or misfiring under load
Not all symptoms point exclusively to a misfire, but the combination of a P0304 code with these symptoms is a strong indicator of cylinder 4 misfire.
Diagnosis and fixes
A structured approach helps identify the exact cause of P0304 in a Honda CR‑V. The steps below outline typical diagnostic and repair paths.
Initial diagnostic steps
Begin with quick, non-invasive checks to confirm a misfire and collect data from live sensors.
- Scan for codes and read freeze‑frame data to confirm cylinder 4 misfire conditions
- Inspect the spark plug for cylinder 4 (color, fouling, gap) and replace if necessary
- Swap the ignition coil from cylinder 4 with another cylinder to see if the misfire moves
- Check spark plug boot and ignition wiring for damage or poor connection
These steps can quickly identify whether the issue is ignition-related or something deeper.
Follow-up tests and fixes
If ignition components check out, proceed with fuel and engine health checks.
- Test or inspect the fuel injector for cylinder 4 (clean, spray pattern, resistance)
- Check for vacuum leaks around intake manifold and hoses near cylinder 4
- Measure compression in cylinder 4 to assess mechanical integrity (piston, rings, valves)
- Verify fuel pressure is correct and consistent with your CR‑V’s specifications
Based on these tests, fixes may include replacing a faulty spark plug or coil, cleaning or replacing the injector, repairing vacuum leaks, or addressing mechanical issues if compression is low.
Optional but prudent steps
In some cases, addressing related systems can prevent future misfires from recurring.
- Update or reprogram the engine control unit if service bulletins apply
- Inspect and clean the EGR valve and passages if applicable
- Inspect timing components if engine timing issues are suspected (less common on modern CR‑V engines)
Professional diagnosis is advised if fuel and ignition checks fail to reveal a clear cause or if mechanical inspection indicates potential engine damage.
When to seek professional help
If you’re unsure about performing electrical or mechanical tests, or if the misfire persists after basic maintenance, consult a qualified mechanic or Honda service center. Misfires can damage the catalytic converter and reduce engine life if ignored, and advanced diagnostic equipment may be required to pinpoint the exact cause.
Summary
P0304 on a Honda CR‑V means the ECU detected a misfire in cylinder 4. The misfire can arise from ignition components, fuel delivery, air intake, or mechanical conditions. Start with a quick inspection of the spark plug and ignition coil, then move to fuel/injection checks and compression testing if needed. Addressing the root cause promptly helps protect the engine and maintain performance.