The ECU detected repeated misfires on cylinder 4 only. Unlike P0300 (random/multiple misfire), P0304 points to a specific cylinder — spark, fuel, or compression on that bore.
Common symptoms
- Rough idle that may feel rhythmic
- Loss of power under load
- Check engine light (steady or flashing)
- Possible fuel smell from unburned fuel
Typical causes
- Worn spark plug or failed ignition coil on cylinder 4
- Clogged or leaking fuel injector for cylinder 4
- Low compression on cylinder 4 (valve, ring, or head gasket)
- Vacuum leak affecting one runner (intake gasket, cracked hose)
- Loose or damaged coil-on-plug boot on cylinder 4
What scanner do you need?
Basic OBD2 code reader (engine codes, live data, smog readiness)
Use live data and freeze frame to confirm the fault before replacing parts. ABS, SRS, and network U-codes usually require a full-system scanner.
Diagnostic tips
- Record freeze frame data when the code first sets
- Clear the code only after fixing the root cause
- Check for related pending codes that point to a common issue
- Verify battery voltage is above 12.4V before chasing sensor codes
